MIPS: mm: Clean up setup of protection map

Protection map difference between RIXI and non RIXI cpus is _PAGE_NO_EXEC
and _PAGE_NO_READ usage. Both already take care of cpu_has_rixi while
setting up the page bits. So we just need one setup of protection map
and can drop the now unused (and broken for RIXI) PAGE_* defines.

Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>