[PARISC] Fix stifb with IOREMAP and a 64-bit kernel

Kill various warnings when built using ioremap.
Remove stifb_{read,write} functions, which are now obsolete (and stack abusers!)
Disable stifb mmap() functionality on a 64-bit kernel, it will crash the
machine.

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