mips: add support for noncached_alloc()

This patch adds support for noncached_alloc() which was only supported by
ARM platform.

Unlike the ARM platform, MMU is not used in u-boot for MIPS. Instead, KSEG
is provided to access uncached memory. So most code of this patch is copied
from cache.c of ARM platform, with only two differences:
1. MMU is untouched in noncached_set_region()
2. Address returned by noncached_alloc() is converted using KSEG1ADDR()

Reviewed-by: Daniel Schwierzeck <daniel.schwierzeck@gmail.com>
Signed-off-by: Weijie Gao <weijie.gao@mediatek.com>