net: dsa: microchip: apply rgmii tx and rx delay in phylink mac config

This patch read the rgmii tx and rx delay from device tree and stored it
in the ksz_port.  It applies the rgmii delay to the xmii tune adjust
register based on the interface selected in phylink mac config. There
are two rgmii port in LAN937x and value to be loaded in the register
vary depends on the port selected.

Signed-off-by: Arun Ramadoss <arun.ramadoss@microchip.com>
Signed-off-by: David S. Miller <davem@davemloft.net>