net: dsa: microchip: ksz8795: move register offsets and shifts to separate struct

In order to get this driver used with other switches the functions need
to use different offsets and register shifts. This patch changes the
direct use of the register defines to register description structures,
which can be set depending on the chips register layout.

Signed-off-by: Michael Grzeschik <m.grzeschik@pengutronix.de>
Signed-off-by: Oleksij Rempel <o.rempel@pengutronix.de>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>