net/phy: support the mv88e6352 switch

This patch add support for the configuration of an external switch from
the 88E6xxx series from Marvell trough an MDIO link using indirect
adressing. This can be used if we do not want to use an EEPROM for the
configuration.

This driver is not generic and was not tested on other switches than the
88e6352. This is proposed as a first implementation that is somewhat
limited but works and that can be used as a basis for further
developments for this switch family.

Signed-off-by: Valentin Longchamp <valentin.longchamp@keymile.com>
cc: Holger Brunck <holger.brunck@keymile.com>
cc: Prafulla Wadaskar <prafulla@marvell.com>
cc: Joe Hershberger <joe.hershberger@gmail.com>