ARM: imx6: DHCOM i.MX6 PDK: Fix usb-otg VBUS regulator

ARM: imx6: DHCOM i.MX6 PDK: Fix usb-otg VBUS regulator

During the conversion of this board to DM_REGULATOR, usb-mass-storage
was broken and started failing with the following error:

        => ums 0 mmc 2
        UMS: LUN 0, dev 2, hwpart 0, sector 0x0, count 0xe90000
        Error enabling VBUS supply
        g_dnl_register: failed!, error: -38
        g_dnl_register failed

Fix this by adding the relevant GPIO to the regulator node.

Fixes: 4ca99fe81aea ("ARM: imx: dh-imx6: Enable DM regulator")
Signed-off-by: Harald Seiler <hws@denx.de>