ArmPkg: Fix writes to GICv3 GICD_IROUTER<n> reg

ArmPkg: Fix writes to GICv3 GICD_IROUTER<n> reg

According to ARM Generic Interrupt Controller Architecture
Specification, GIC architecture version 3.0 and version 4.0,
GICD_IROUTER<n> is a 64-bit register.

Fixed code to use 64 bit MMIO write operations so that the
Aff3 value (bits [39:32]) is written to GICD_IROUTER<n>.

Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Sami Mujawar <sami.mujawar@arm.com>
Reported-by: Carl van Schaik <carl@cog.systems>
Reviewed-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>