ahc(4): Default to memory mapped IO

[hpvb]

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.