net: dsa: microchip: add DSA support for microchip LAN937x

Basic DSA driver support for lan937x and the device will be
configured through SPI interface.
It adds the lan937x_dev_ops in ksz_common.c file and tries to reuse the
functionality of ksz9477 series switch.

drivers/net/dsa/microchip/ path is already part of MAINTAINERS &
the new files come under this path. Hence no update needed to the
MAINTAINERS

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