ARM: mvebu: remove duplicated regulator definition in Armada 388 GP

The Armada 388 GP Device Tree file describes two times a regulator
named 'reg_usb2_1_vbus', with the exact same description. This has
been wrong since Armada 388 GP support was introduced.

Fixes: 928413bd ("ARM: mvebu: Add Armada 388 General Purpose Development Board support")
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Cc: <stable@vger.kernel.org> # v4.0+
Signed-off-by: Gregory CLEMENT <gregory.clement@free-electrons.com>