net: dsa: microchip: common ksz_spi_probe for ksz switches

As of now, there are two spi probes, one ksz8795_spi.c and other
ksz9477_spi.c. This patch combines two files into single ksz_spi.c. The
difference between the two are regmap config and struct ksz8. The regmap
config is assigned based on the platform data. And struct ksz8 is left
untouched, as it is used only ksz8795.c. It can be used for all
other switches also in future.

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