[PARISC] Add dummy isa_(bus|virt)_to_(virt|bus) inlines

Less painful than fixing up the Kconfig for a pile of drivers to only build
on X86 && ARM && MIPS...

Just make them BUG(), as defining them to be 1:1 with physical memory will
likely HPMC the box anyways.

Signed-off-by: Kyle McMartin <kyle@parisc-linux.org>