arm64: dts: imx8mp-evk: Add supports-clkreq property to PCIe M.2 port

According to PCIe r6.1, sec 5.5.1.

The following rules define how the L1.1 and L1.2 substates are entered:
Both the Upstream and Downstream Ports must monitor the logical state of
the CLKREQ# signal.

Typical implement is using open drain, which connect RC's clkreq# to
EP's clkreq# together and pull up clkreq#.

imx8mp-evk matches this requirement, so add supports-clkreq to allow
PCIe device enter ASPM L1 Sub-State.

Signed-off-by: Richard Zhu <hongxing.zhu@nxp.com>
Reviewed-by: Frank Li <Frank.Li@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>

arch/arm64/boot/dts/freescale/imx8mp-evk.dts

@@ -710,6 +710,7 @@
 	pinctrl-0 = <&pinctrl_pcie0>;
 	reset-gpio = <&gpio2 7 GPIO_ACTIVE_LOW>;
 	vpcie-supply = <&reg_pcie0>;
+	supports-clkreq;
 	status = "okay";
 };