dt-bindings: regulator: qcom,usb-vbus-regulator: Mark reg as required

The regulator code needs to know the location of the register to write to
to switch on/off. Right now we have a driver that does this, a yaml that
partially describes it and no dts that uses it.

Switching on the VBUS for sm8250 shows that we haven't documented reg as a
required property, do so now.

Acked-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Bryan O'Donoghue <bryan.odonoghue@linaro.org>
Link: https://lore.kernel.org/r/20230515133643.3621656-2-bryan.odonoghue@linaro.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>