usb: r8a66597: Add optional DM VBUS regulator support

Add DM regulator support for toggling VBUS, this is useful on boards
which control the VBUS e.g. through GPIO.

Signed-off-by: Marek Vasut <marek.vasut+renesas@gmail.com>
Cc: Chris Brandt <chris.brandt@renesas.com>