drivers: ethernet: vsc8541: add RGMII clock delay configuration

As the code noted, the RGMII RX and TX clock delay values may need to
change depending on the MAC configuration or the PCB layout. Add
properties to allow configuring these in the device tree, defaulting to
the previous hard-coded values if not present.

Signed-off-by: Robert Hancock <robert.hancock@calian.com>