VBUS is supplied by an external regulator controlled by a GPIO pin. This
patch models the regulator as regulator-usb3-vbus.

Cc: Andy Gross <agross@kernel.org>
Cc: Bjorn Andersson <bjorn.andersson@linaro.org>
Cc: Rob Herring <robh+dt@kernel.org>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: linux-arm-msm@vger.kernel.org
Cc: devicetree@vger.kernel.org
Cc: linux-kernel@vger.kernel.org
Tested-by: Bjorn Andersson <bjorn.andersson@linaro.org>
Signed-off-by: Bryan O'Donoghue <bryan.odonoghue@linaro.org>
Link: https://lore.kernel.org/r/20200311191517.8221-5-bryan.odonoghue@linaro.orgSigned-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>