arm64: dts: imx8mn: Enable Overdrive mode

The i.MX8M Nano supports and overdrive mode if the SoC is given
the proper voltage.  Add imx8mn-overdrive.dtsi file which can
be included by boards who support the voltage necessary to handle
the faster clocks.  This increases the GPU clocks from 400MHz to
600MHz.
Signed-off-by: Adam Ford <aford173@gmail.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>

arch/arm64/boot/dts/freescale/imx8mn-overdrive.dtsi

+// SPDX-License-Identifier: (GPL-2.0+ OR MIT)
+
+&gpu {
+	assigned-clocks = <&clk IMX8MN_CLK_GPU_CORE>,
+			  <&clk IMX8MN_CLK_GPU_SHADER>,
+			  <&clk IMX8MN_CLK_GPU_AXI>,
+			  <&clk IMX8MN_CLK_GPU_AHB>,
+			  <&clk IMX8MN_GPU_PLL>;
+	assigned-clock-parents = <&clk IMX8MN_GPU_PLL_OUT>,
+				  <&clk IMX8MN_GPU_PLL_OUT>,
+				  <&clk IMX8MN_SYS_PLL1_800M>,
+				  <&clk IMX8MN_SYS_PLL1_800M>;
+	assigned-clock-rates = <600000000>,
+			       <600000000>,
+			       <800000000>,
+			       <400000000>,
+			       <1200000000>;
+};