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>