dt-bindings: phy: Add a new property drive-impedance-ohm for RK's emmc PHY

This patch documents the new proprty drive-impedance-ohm for
Rockchip's eMMC PHY node.

Signed-off-by: Christoph Muellner <christoph.muellner@theobroma-systems.com>
Signed-off-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>
Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>