MIPS: Add kernel_pref & user_pref helpers

Add kernel_pref & user_pref macros to asm/asm-eva.h, providing an
abstraction around EVA & non-EVA pref instructions consistent with the
existing macros we have for cache & load/store instructions.

Signed-off-by: Paul Burton <paul.burton@mips.com>
Patchwork: https://patchwork.linux-mips.org/patch/20906/
Cc: linux-mips@linux-mips.org