Platform/Loongson: Add QemuFwCfgLib.

QemuFwCfgLib for PEI phase.
This library obtains the QemuFWCfg base address by
directly parsing the fdt, and reads and writes the data
in the QemuFWCfg by operating on the QemuFWCfg base address.

REF: https://bugzilla.tianocore.org/show_bug.cgi?id=4054



Cc: Ard Biesheuvel <ardb+tianocore@kernel.org>
Cc: Bibo Mao <maobibo@loongson.cn>
Cc: Chao Li <lichao@loongson.cn>
Cc: Leif Lindholm <quic_llindhol@quicinc.com>
Cc: Liming Gao <gaoliming@byosoft.com.cn>
Cc: Michael D Kinney <michael.d.kinney@intel.com>
Signed-off-by: xianglai li <lixianglai@loongson.cn>
Reviewed-by: Chao Li <lichao@loongson.cn>