MIPS: Add Loongson cpu-feature-overrides.h

I have taken Wu Zhangjin's and Philippe Vachon's version as references,
did a little modification and tested on 16K page size kernel. It works
well.

Unfornately although it already has defined cpu_has_dc_aliases as 1, 4k
page size still not working.  More work needed here.

Signed-off-by: Zhang Le <r0bertz@gentoo.org>
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>