xtensa: support aliasing cache in k[un]map_atomic

Map high memory pages at virtual addresses with color that match color
of their physical address. Existing cache alias management mechanisms
may be used with such pages.

Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>

diff --cc arch/xtensa/include/asm/fixmap.h
Simple merge

diff --cc arch/xtensa/include/asm/page.h
Simple merge

diff --cc arch/xtensa/mm/highmem.c
Simple merge