LoongArch: kdump: Add crashkernel=YM handling

When the kernel crashkernel parameter is specified with just a size,
we are supposed to allocate a region from RAM to store the crashkernel,
"crashkernel=512M" would be recommended for kdump.

Fix this by lifting similar code from x86, importing it to LoongArch
with LoongArch specific parameters added. We allocate the crashkernel
region from the first 4GB of physical memory (because SWIOTLB should be
allocated below 4GB). However, LoongArch currently does not implement
crashkernel_low and crashkernel_high the same as x86.

When X is not specified, crash_base defaults to 0 (crashkernel=YM@XM).

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