[PARISC] Fix and cleanup ioremap.c to work with 4level-fixup.h

Fixup ioremap a bit. It seems to work on 32-bit kernels, but fails
miserably on the first ioremapped access on 64-bit kernels. Also, having
STI enabled causes it to fail. Probably because we're passing an ioremapped
region to a real-mode STI call...

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