arm64: dts: rockchip: Define drive-impedance-ohm for RK3399's emmc-phy.

A previous patch introduced the property 'drive-impedance-ohm'
for the RK3399's emmc phy node. This patch sets this value
explicitly to the default value of 50 Ohm.
Signed-off-by: Christoph Muellner <christoph.muellner@theobroma-systems.com>
Signed-off-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>
Signed-off-by: Heiko Stuebner <heiko@sntech.de>

arch/arm64/boot/dts/rockchip/rk3399.dtsi

@@ -1457,6 +1457,7 @@ pcie_phy: pcie-phy {
 			clock-names = "refclk";
 			#phy-cells = <1>;
 			resets = <&cru SRST_PCIEPHY>;
+			drive-impedance-ohm = <50>;
 			reset-names = "phy";
 			status = "disabled";
 		};