net: dwc_eth_qos: Add i.MX8M Plus RMII support

With DM clock support in place, it is easy to add RMII support into the
MAC driver. The RMII cannot operate at 1000 Mbps and at 100 and 10 Mbps
the clock frequency is 50 MHz and 5 MHz instead of 25 MHz and 2.5 MHz.

The board DT requires the following adjustments to EQoS node:
  phy-mode = "rmii";
  assigned-clock-parents = <&clk IMX8MP_SYS_PLL1_266M>,
  			<&clk IMX8MP_SYS_PLL2_100M>,
  			<&clk IMX8MP_SYS_PLL2_50M>;
  assigned-clock-rates = <0>, <100000000>, <50000000>;

Reviewed-by: Ramon Fried <rfried.dev@gmail.com>
Signed-off-by: Marek Vasut <marex@denx.de>