Copy head (r256279) to stable/10 as part of the 10.0-RELEASE cycle.

Approved by: re (implicit)
Sponsored by: The FreeBSD Foundation

- Implement iclear methods for QUICC and SAB 82532. With r253161 in place,
this is is crucial at least for the latter.
What happens is that attaching uart(4) to scc(4) causes the SAB 82532 to
"receive" something and trigger a SER_INT_RXREADY interrupt, given that
at least fast/filter interrupts are already enabled. Prior to r253161,
uart_bus_ihand() was set up at this point and handled that condition,
i. e. read the RX FIFO and issued a Receive Message Complete.
Now, uart_bus_ihand() and uart_intr() are setup after attaching uart(4),
leaving the SER_INT_RXREADY interrupt triggered during the latter to
be handled by the iclear method. However, with that method not implement,
this in turn causes SAB 82532 to not issue any further SER_INT_RXREADY
interrupts until the RX FIFO is full again. Thus, 15 received bytes go
to nowhere, given that "the other half" of the RX FIFO is used for status
information. Hence, implementing sab82532_bfe_iclear() fixes things again.
Potentially, the same problem exists for QUICC.
- Remove unnecessary __RMAN_RESOURCE_VISIBLE.
- Remove a superfluous header.
- Use KOBJMETHOD_END.
- Mark unused arguments as such.
- Remove variables unused after initialization.

Reviewed by: marcel (earlier version)

Add a MacIO bus attachment. The Z8530 as present in the Mac needs
a different register shift and is fed by a different clock than
we use for UltraSPARC hardware. To deal with this, the regshft and
rclk fields in the class structure are removed and bus frontends
now pass the right regshft and rclk to the probe function where
they're put in the BAS and passed in to subordinate drivers.

Add scc(4), a driver for serial communications controllers. These
controllers typically have multiple channels and support a number
of serial communications protocols. The scc(4) driver is itself
an umbrella driver that delegates the control over each channel
and mode to a subordinate driver (like uart(4)).
The scc(4) driver supports the Siemens SAB 82532 and the Zilog
Z8530 and replaces puc(4) for these devices.