arm/arm64: KVM: vgic: Fix SGI writes to GICD_I{CS}PENDR0

Writes to GICD_ISPENDR0 and GICD_ICPENDR0 ignore all settings of the
pending state for SGIs.  Make sure the implementation handles this
correctly.

Signed-off-by: Christoffer Dall <christoffer.dall@linaro.org>