kernel/i386: add pmap_invalidate* functions

They are roughly similar to the amd64 implementations but due to
i386 pmap shortcomings we have to completely flush cpu caches in
some cases, which is not ideal performance-wise.
kernel/i386: Add PAT support, mapdev_attr, kmem_alloc_attr
kernel/i386: Implement pmap_mapbios()
vga_pci: Add API to map the Video BIOS

Obtained-from: FreeBSD
SVN commit r254882 and a few missing bits