phy: marvell: a3700: Update compatible string to official DT bindings

In commit d368e107 ("phy: marvell: a3700: Convert to official DT
bindings in COMPHY driver") was done update to official DT bindings but
compatible string of official DT bindings was not updated.

Fix it now.

Fixes: d368e107 ("phy: marvell: a3700: Convert to official DT bindings in COMPHY driver")
Signed-off-by: Pali Rohár <pali@kernel.org>
Reviewed-by: Stefan Roese <sr@denx.de>