ArmPkg/ArmGicLib: Fix GICR_IPRIORITYR address wrong issue

ArmPkg/ArmGicLib: Fix GICR_IPRIORITYR address wrong issue

The register address of GICR_IPRIORITYR is in SGI_base frame. Add
IPRIORITY_ADDRESS macro for getting GICR_IPRIORITYR address. Otherwise
GIC RAS error(Uncorrected software error) may report in ArmGicDxe.

This resolves BZ: https://bugzilla.tianocore.org/show_bug.cgi?id=3236

Signed-off-by: Ming Huang <huangming@linux.alibaba.com>
Reviewed-by: Leif Lindholm <leif@nuviainc.com>
Tested-by: Ard Biesheuvel <ardb@kernel.org> # QEMU/kvm guest on ThunderX2
Tested-by: Quan Nguyen <quan@os.amperecomputing.com>