usb: phy: tegra: Use generic stub for a missing VBUS regulator

Regulator core provides dummy regulator if device-tree doesn't define VBUS
regulator.

Signed-off-by: Dmitry Osipenko <digetx@gmail.com>
Link: https://lore.kernel.org/r/20200106013416.9604-10-digetx@gmail.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>