MdePkg/BaseLib: RISC-V: Add few more helper functions

REF: https://bugzilla.tianocore.org/show_bug.cgi?id=4076

Few of the basic helper functions required for any
RISC-V CPU were added in edk2-platforms. To support
qemu virt, they need to be added in BaseLib.

Cc: Michael D Kinney <michael.d.kinney@intel.com>
Cc: Liming Gao <gaoliming@byosoft.com.cn>
Cc: Zhiguang Liu <zhiguang.liu@intel.com>
Cc: Daniel Schaefer <git@danielschaefer.me>
Signed-off-by: Sunil V L <sunilvl@ventanamicro.com>
Acked-by: Abner Chang <abner.chang@amd.com>
Reviewed-by: Andrei Warkentin <andrei.warkentin@intel.com>
Reviewed-by: Michael D Kinney <michael.d.kinney@intel.com>

MdePkg/Library/BaseLib/BaseLib.inf

@@ -401,6 +401,9 @@
   RiscV64/RiscVCpuPause.S           | GCC
   RiscV64/RiscVInterrupt.S          | GCC
   RiscV64/FlushCache.S              | GCC
+  RiscV64/CpuScratch.S              | GCC
+  RiscV64/ReadTimer.S               | GCC
+  RiscV64/RiscVMmu.S                | GCC
 
 [Sources.LOONGARCH64]
   Math64.c