ArmPkg/ArmLib: rename AArch64 variant of ArmReadIdPfr0

ArmReadIdPfr0 is now used only inside ArmLib. Rename the AArch64
variant ArmReadIdAA64Pfr0 and add a declaration of that only into
local header AArch64/AArch64Lib.h.

Cc: Ard Biesheuvel <ard.biesheuvel@arm.com>
Signed-off-by: Leif Lindholm <leif@nuviainc.com>
Reviewed-by: Ard Biesheuvel <ard.biesheuvel@arm.com>

ArmPkg/Library/ArmLib/AArch64/AArch64Lib.c

@@ -85,5 +85,5 @@ ArmHasGicSystemRegisters (
   VOID
   )
 {
-  return ((ArmReadIdPfr0 () & AARCH64_PFR0_GIC) != 0);
+  return ((ArmReadIdAA64Pfr0 () & AARCH64_PFR0_GIC) != 0);
 }