mvebu: usb: xhci: Add VBUS regulator supply to the host driver

The USB device should linked to VBUS regulator through "vbus-supply"
DTS property.
This patch adds handling for "vbus-supply" property inside the USB
device entry for turning on the VBUS regulator upon the host adapter probe.

Signed-off-by: Konstantin Porotchkin <kostap@marvell.com>
Cc: Stefan Roese <sr@denx.de>
Cc: Marek Vasut <marex@denx.de>
Cc: Nadav Haklai <nadavh@marvell.com>
Cc: Neta Zur Hershkovits <neta@marvell.com>
Cc: Igal Liberman <igall@marvell.com>
Cc: Haim Boot <hayim@marvell.com>
Acked-by: Marek Vasut <marex@denx.de>
Signed-off-by: Stefan Roese <sr@denx.de>