ArmPkg/TimerDxe: Add ISB for timer compare value reload

ArmPkg/TimerDxe: Add ISB for timer compare value reload

If timer interrupt is level sensitive, reloading timer compare
register has a side effect of clearing GIC pending status, so a "ISB"
is needed to make sure this instruction is executed before enabling
CPU IRQ, or else we may get spurious timer interrupts.

Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Heyi Guo <heyi.guo@linaro.org>
Acked-by: Marc Zyngier <marc.zyngier@arm.com>
Reviewed-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>