arm64: dts: add msm8996 compatible to gicv3

Add compatible to gicv3 node to enable quirk required to restrict writing
to GICR_WAKER register which is restricted on msm8996 SoC in Hypervisor.

With this quirk MSM8996 can at least boot out of mainline, which can help
community to work with boards based on MSM8996.

Without this patch Qualcomm DB820c board reboots on mainline.

Signed-off-by: Srinivas Kandagatla <srinivas.kandagatla@linaro.org>
Signed-off-by: Andy Gross <andy.gross@linaro.org>