Fix existing and add missing PT_MIPS_XXX constants in elf.h.
This is copied from kernel header arch/mips/include/asm/elf.h.

Reviewed-by: Aleksandar Markovic <amarkovic@wavecomp.com>
Signed-off-by: Stefan Markovic <smarkovic@wavecomp.com>
Signed-off-by: Aleksandar Markovic <amarkovic@wavecomp.com>