if_dwc_rk: Add DesignWare driver for RockChip SoCs.

Add driver for the designware ethernet controller found in some RockChip SoCs.
The driver still rely on a lot of things setup by the bootloader like clocks
and phy mode.
But since netbooting is the only/easiest way to boot rockchip board at the
moment add the driver so other people can test/dev on thoses boards.