net: Micrel KSZ8864RMN 4-port managed switch support

This patch adds support for the Micrel KSZ8864RMN switch to the spi_ks8995
driver. The KSZ8864RMN switch has a wider 256-byte register space.

Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>
Signed-off-by: David S. Miller <davem@davemloft.net>