MFC r256994, r257016, r257055, r257059, r257060, r257075

Add two new interfaces to ofw_bus:
- ofw_bus_map_intr()
Maps an (iparent, IRQ) tuple to a system-global interrupt number in some
platform dependent way. This is meant to be implemented as a replacement
for [FDT_]MAP_IRQ() that is an MI interface that knows about the bus
hierarchy.
- ofw_bus_config_intr()
Configures an interrupt (previously mapped) based on firmware sense flags.
This replaces manual interpretation of the sense field in bus drivers and
will, in a follow-up, allow that interpretation to be redirected to the PIC
drivers where it belongs. This will eventually replace the tables in
/sys/dev/fdt/fdt_ARCH.c

The PowerPC/AIM code has been converted to use these globally, with an
implementation in terms of MAP_IRQ() and powerpc_config_intr(), assuming
OpenPIC, at the bus root in nexus(4). The ofw_bus_config_intr() will shortly
be integrated into pic_if.m and bounced through nexus into the PIC tree.

Factor out MI portions of the PowerPC nexus device into /sys/dev/ofw. The
sparc64 driver will be modified to use this shortly.

Allow PIC drivers to translate firmware sense codes for themselves. This
is designed to replace the tables in dev/fdt/fdt_ARCH.c, but will not
happen quite yet.

Do not map IRQs twice. This fixes PowerPC/FDT systems with multiple PICs,
which would try to treat the previously-mapped interrupts from
fdt_decode_intr() as interrupt line numbers on the same parent PIC.

Remove some of the code required for supporting ssm(4) on SPARC in favor
of a more PowerPC/FDT-focused design. Whenever SPARC64 is integrated
into this rework, this should be (trivially) revisited.

MFC r256994, r257016, r257055, r257059, r257060, r257075

Add two new interfaces to ofw_bus:
- ofw_bus_map_intr()
Maps an (iparent, IRQ) tuple to a system-global interrupt number in some
platform dependent way. This is meant to be implemented as a replacement
for [FDT_]MAP_IRQ() that is an MI interface that knows about the bus
hierarchy.
- ofw_bus_config_intr()
Configures an interrupt (previously mapped) based on firmware sense flags.
This replaces manual interpretation of the sense field in bus drivers and
will, in a follow-up, allow that interpretation to be redirected to the PIC
drivers where it belongs. This will eventually replace the tables in
/sys/dev/fdt/fdt_ARCH.c

The PowerPC/AIM code has been converted to use these globally, with an
implementation in terms of MAP_IRQ() and powerpc_config_intr(), assuming
OpenPIC, at the bus root in nexus(4). The ofw_bus_config_intr() will shortly
be integrated into pic_if.m and bounced through nexus into the PIC tree.

Factor out MI portions of the PowerPC nexus device into /sys/dev/ofw. The
sparc64 driver will be modified to use this shortly.

Allow PIC drivers to translate firmware sense codes for themselves. This
is designed to replace the tables in dev/fdt/fdt_ARCH.c, but will not
happen quite yet.

Do not map IRQs twice. This fixes PowerPC/FDT systems with multiple PICs,
which would try to treat the previously-mapped interrupts from
fdt_decode_intr() as interrupt line numbers on the same parent PIC.

Remove some of the code required for supporting ssm(4) on SPARC in favor
of a more PowerPC/FDT-focused design. Whenever SPARC64 is integrated
into this rework, this should be (trivially) revisited.