Document bindings for Marvell Aquantia PHY.

The Marvell Aquantia PHY require a firmware to work correctly and there
at least 3 way to load this firmware.

Describe all the different way and document the binding "firmware-name"
to load the PHY firmware from userspace.
Signed-off-by: Christian Marangi <ansuelsmth@gmail.com>
Reviewed-by: Conor Dooley <conor.dooley@microchip.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>