Documentation: LoongArch: Add basic documentations

Add some basic documentation for LoongArch. LoongArch is a new RISC ISA,
which is a bit like MIPS or RISC-V. LoongArch includes a reduced 32-bit
version (LA32R), a standard 32-bit version (LA32S) and a 64-bit version
(LA64).

Tested-by: Bagas Sanjaya <bagasdotme@gmail.com>
Reviewed-by: Jiaxun Yang <jiaxun.yang@flygoat.com>
Co-developed-by: WANG Xuerui <git@xen0n.name>
Signed-off-by: WANG Xuerui <git@xen0n.name>
Signed-off-by: Huacai Chen <chenhuacai@loongson.cn>