arm64: add a driver for the uart found on Apple Silicon machines

This is a revival of the old exynos4210 driver, with some additional
bits to configure the apple "s5l" uart (which is actually slightly
different to operate).

This hasn't been tested on anything that would hit the non-s5l path, so
banish it off to the apple/ domain until someone cares to confirm that
none of the other hardware is broken -- it may be that nobody does, but
the complexity isn't too bad: mostly the driver1 construct added to the
uart_bas that we use to avoid having a whole bunch of shims for uart
driver methods and hardcoded references to the cfg structs.

Reviewed by: andrew
Differential Revision: https://reviews.freebsd.org/D48120