dt-bindings: clock: imx8m: Add GIC clock

This should be defined in the clock tree so that parents are not
shutdown by accident

Signed-off-by: Leonard Crestez <leonard.crestez@nxp.com>
Reviewed-by: Abel Vesa <abel.vesa@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>