ArmPkg: Add helper to read the Memory Model Features Register 2

Add helper function to read the MMFR2 register. We will need this to
determine CCIDX support.

Signed-off-by: Rebecca Cran <rebecca@nuviainc.com>
Reviewed-by: Leif Lindholm <leif@nuviainc.com>
Reviewed-by: Sami Mujawar <sami.mujawar@arm.com>

ArmPkg/Library/ArmLib/AArch64/AArch64Lib.h

@@ -41,5 +41,16 @@ EFIAPI
 ArmReadIdAA64Pfr0 (
   VOID
   );
+
+/** Reads the ID_AA64MMFR2_EL1 register.
+
+   @return The contents of the ID_AA64MMFR2_EL1 register.
+**/
+UINTN
+EFIAPI
+ArmReadIdAA64Mmfr2 (
+  VOID
+  );
+
 #endif // __AARCH64_LIB_H__