ARM: dts: i.MX51: digi-connectcore-som: Setup usbotg vbus-supply

The VBUS on usbotg is connected to the PMIC SWBST, let's reflect
that in the bindings.

Signed-off-by: Alexander Shiyan <eagle.alexander923@gmail.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>