phy: marvell: a3700: Fix compatible string for ehci

The DTS file for armada-37xx uses the string "marvell,armada3700-ehci",
but the code searched for "marvell,armada-3700-ehci".

Signed-off-by: Marek Behun <marek.behun@nic.cz>
Signed-off-by: Stefan Roese <sr@denx.de>