arm64: dts: imx8mp: adjust GIC CPU mask to match number of CPUs

i.MX 8M Plus has four Cortex-A CPUs, not six.  Using higher value is
harmless but adjust it to match real HW.

Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>
Reviewed-by: Jacky Bai <ping.bai@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>