MIPS: Loongson: Disallow 4kB pages

Currently, with PAGE_SIZE_4KB, the kernel for loongson will hang on:

Kernel panic - not syncing: Attempted to kill init!

The possible reason is the cache aliases problem:

Loongson 2F has 64kb, 4 way L1 Cache, the way size is 16kb, which is bigger
then 4kb. so, If using 4kb page size, there is cache aliases problem.
To avoid this kind of problem, extra cache flushing.  The 2nd possible
solution is 16kb page size which avoids cache aliases without the need for
extra cache flushes.  So we disable 4kB pages until the aliasing issue is
solved.

Signed-off-by: Wu Zhangjin <wuzhangjin@gmail.com>
Patchwork: http://patchwork.linux-mips.org/patch/736/
Cc: linux-mips@linux-mips.org
Cc: zhangfx@lemote.com
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>