mips: use asm-generic/unaligned.h

Mips essentially duplicates the content of asm-generic/unaligned.h, so use
that file directly instead.

Signed-off-by: Jens Wiklander <jens.wiklander@linaro.org>
Reviewed-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>