usb: Add EHCI support for Armada 38x (mvebu)

This patch adds USB EHCI host support for the common mvebu platform.
Including the Armada 38x.

Tested on DB-88F6280-GP eval board.

Signed-off-by: Stefan Roese <sr@denx.de>
Reviewed-by: Marek Vasut <marex@denx.de>
Cc: Luka Perkov <luka.perkov@sartura.hr>