Improve periodic USB transfers (device intr, isoc) used for input
devices, audio, and video. It's still not perfect, and will need
further improvements.

High level, the diff contains following changes:

* Sync up with the Linux code base, which did re-work the periodic
scheduling code path.
* Run the driver in IPL_VM instead of IPL_USB to prioritize us before
lower/equal interrupts (same what NetBSD does).
* Add two new flags to our USB stack required by the updated driver
code:
- 'multi' flag in the usbd_hub structure to keep track whether
a hub has one Transaction Translator for all ports (single TT)
or one Transaction Translator per port (multi TT).
- 'hcpriv' pointer in the usbd_tt structure for the HC driver to
allocate memory for the scheduling depending on single or multi TT.

"go for it" kettenis@

The updated dwctwo(4) driver introduces two new parameters.

arm_intr_establish_fdt() has long been renamed to fdt_intr_establish().

The dwctwo(4) FDT glue is now identical on armv7 and arm64. Move it to
/dev/fdt.

ok patrick@, visa@

The updated dwctwo(4) driver introduces two new parameters.

arm_intr_establish_fdt() has long been renamed to fdt_intr_establish().

The dwctwo(4) FDT glue is now identical on armv7 and arm64. Move it to
/dev/fdt.

ok patrick@, visa@

arm_intr_establish_fdt() has long been renamed to fdt_intr_establish().

The dwctwo(4) FDT glue is now identical on armv7 and arm64. Move it to
/dev/fdt.

ok patrick@, visa@

The dwctwo(4) FDT glue is now identical on armv7 and arm64. Move it to
/dev/fdt.

ok patrick@, visa@