ArmPkg/ArmGicDxe ARM: fix encoding for GICv3 interrupt acknowledge

ArmPkg/ArmGicDxe ARM: fix encoding for GICv3 interrupt acknowledge

Fix a typo in the 32-bit ARM version of the GICv3 driver, which uses
the wrong system register encoding to access ICC_IAR1, and attempted
to access ICC_IAR0 instead. This results in boot time hangs both
under QEMU emulation and on real hardware.

Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Reviewed-by: Leif Lindholm <leif.lindholm@linaro.org>