ahc(4): Default to memory mapped IO

When this driver was written it made sense to make this default to off,
but these days almost all BIOSses will do the right thing. Furthermore
non-mmio communication only works on Intel architectures.

So lets default to allowing mmio, but not change the semantics of the
AHC_ALLOW_MEMIO flag to not break existing installs. Also document the
already existing hint.ahc.<unit>.allow_memio.

Signed-off-by: HP van Braam <hp@tmm.cx>
Reviewed by: imp (small style tweak)
Pull Request: https://github.com/freebsd/freebsd-src/pull/1219