LoongArch: Support access filter to /dev/mem interface

Accidental access to /dev/mem is obviously disastrous, but specific
access can be used by people debugging the kernel. So select GENERIC_
LIB_DEVMEM_IS_ALLOWED, as well as define ARCH_HAS_VALID_PHYS_ADDR_RANGE
and related helpers, to support access filter to /dev/mem interface.

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