MFC r268880:
Allow mappings of memory not previously direct-mapped by the kernel when
calling mmap on /dev/mem and add a handler for the possible userland
machine checks that may result. Remove some pointless and wrong copy/paste
that has been in here for a decade as well.

This results in a /dev/mem with identical semantics to the x86 version.
MFC r265883,268898:

Repair bitrot in PS3 memory and interrupt allocation.
MFC r268350 (nwhitehorn):

  Use common vt_fb parts in ofwfb as far as we are able without
  sacrificing performance.

Sponsored by:	The FreeBSD Foundation
MFC r269278 (nwhitehorn):

  Make mmap() of the console device when using ofwfb work like other
  supported framebuffer drivers. This lets ofwfb work with
  xf86-video-scfb and makes the driver much more generic and less
  PCI-centric. This changes some user-visible behavior and will require
  updates to the xorg-server port on PowerPC when using ATI graphics
  cards.

Relnotes:	Yes
Sponsored by:	The FreeBSD Foundation