net: aquantia: add support for Phy access

GPIO PIN control and access is done by direct phy manipulation.
Here we add an aq_phy module which is able to access phy registers
via MDIO access mailbox.

Access is controlled via HW semaphore.

Co-developed-by: Nikita Danilov <nikita.danilov@aquantia.com>
Signed-off-by: Nikita Danilov <nikita.danilov@aquantia.com>
Signed-off-by: Dmitry Bezrukov <dmitry.bezrukov@aquantia.com>
Signed-off-by: Igor Russkikh <igor.russkikh@aquantia.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>