arm64: dts: imx8mm: drop redundant interrupt-parent

There is interrupt-parent = <&gic> in root node, there is no
need set it again in node ddr-pmu@3d800000.

Signed-off-by: Peng Fan <peng.fan@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>