U5SeriesPkg/FreedomU540HiFiveUnleashedBoard: Support MMU with SV39/48…

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By following https://github.com/riscv-admin/riscv-uefi-edk2-docs to build edk2 for riscv qemu(u540). build -a RISCV64 -p Platform/SiFive/U5SeriesPkg/FreedomU540HiFiveUnleashedBoard/U540.dsc -t GCC5 it will occur build error like :

It will build success after add missing library class.