phy: marvell: a3700: Convert to official DT bindings in COMPHY driver

Convert A3720 common PHY driver to official DT bindings.

This puts us closer to be able to synchronize A3720 device-trees with
those from Linux.

Signed-off-by: Pali Rohár <pali@kernel.org>
Signed-off-by: Marek Behún <marek.behun@nic.cz>
Cc: Konstantin Porotchkin <kostap@marvell.com>
Cc: Robert Marko <robert.marko@sartura.hr>
Cc: Luka Perkov <luka.perkov@sartura.hr>
Cc: Marcin Wojtas <mw@semihalf.com>
Cc: Grzegorz Jaszczyk <jaz@semihalf.com>
Reviewed-by: Stefan Roese <sr@denx.de>