arm: mvebu: dts: turris_mox: fix non-working USB port

Commit 0934dddc ("arm: a37xx: Update DTS files to version from
upstream Linux kernel") ported Linux's device-tree files for Armada 3720
SOCs. This broke USB port on Turris MOX, because in Linux' DTS the bus
voltage supply is described as a `phy-supply` property of connector
node, a mechanism that is not supported in U-Boot yet.

For now, fix this by adding `vbus-supply` to usb3 node.

Fixes: 0934dddc ("arm: a37xx: Update DTS files to version from upstream Linux kernel")
Signed-off-by: Marek Behún <marek.behun@nic.cz>
Reviewed-by: Stefan Roese <sr@denx.de>