net: dsa: microchip: lan937x: add phy read and write support

This patch add support for the writing and reading of the phy registers.
LAN937x uses the Vphy indirect addressing method for accessing the phys.

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