net: dwc_eth_qos: Add support for st, ext-phyclk property

net: dwc_eth_qos: Add support for st, ext-phyclk property

The "st,ext-phyclk" property is a unification of "st,eth-clk-sel"
and "st,eth-ref-clk-sel" properties. All three properties define
ETH CK clock direction, however:
- "st,eth-clk-sel" selects clock direction for GMII/RGMII mode
- "st,eth-ref-clk-sel" selects clock direction for RMII mode
- "st,ext-phyclk" selects clock direction for all RMII/GMII/RGMII modes
The "st,ext-phyclk" is the preferrable property to use.

Signed-off-by: Marek Vasut <marex@denx.de>
Reviewed-by: Christophe ROULLIER <christophe.roullier@foss.st.com>