asmc: try PIO before MMIO to avoid false T2 detection

Add hw.asmc.system-state and hw.asmc.board-id read-only sysctls to
expose the T2 system state register and Mac board identifier via SMC.

Try PIO access before MMIO during probe to prevent false T2 detection
on Macs that happen to have something mapped at the T2 BAR address.

Reviewed by: adrian
Differential Revision: https://reviews.freebsd.org/D57844