ARM: dts: rockchip: Remove "regulator-always-on" in vcc_rmii for Radxa Rock

On Rockchip RK3188 SoCs the platform driver emac_rockchip is used. This variant driver
enables this regulator when the device driver is loaded. The phy no longer needs
to be always on.

Signed-off-by: Romain Perier <romain.perier@gmail.com>
Signed-off-by: Heiko Stuebner <heiko@sntech.de>