arm: mvebu: add multiple usb-hostcontroller support for AXP

This patch adds support for multiple hostcontrollers to the ehci-marvell driver
and enables all 3 usb-hcs on the db-mv784mp-gp board.

It depends on the initial Armada XP usb support patch from Stefan.

Signed-off-by: Anton Schubert <anton.schubert@gmx.de>
Reviewed-by: Stefan Roese <sr@denx.de>
Cc: Luka Perkov <luka.perkov@sartura.hr>