LoongArch: Allow specify SIMD width via kernel parameters

LoongArch: Allow specify SIMD width via kernel parameters

For power saving or debugging purpose, we usually want to limit the SIMD
(LSX/LASX) usage on a rich feature platform. So allow specify SIMD width
via kernel parameters "simd=".

Allowed values of "simd=" are any integers, and recommended values are:
0:		Disable all SIMD features;
128:		Enable at most 128bit SIMD features;
256:		Enable at most 256bit SIMD features;
-1(default):	Enable as many as possible SIMD features automatically.

Signed-off-by: Huacai Chen <chenhuacai@loongson.cn>