regulator: Add dt-binding for QCOM PMIC VBUS output regulator

This describes how to enable the Qualcomm PMIC VBUS booster used for
providing power to connected USB peripherals when the USB role is host
mode.  The driver itself will register the vbus_usb regulator, so that
external drivers can utilize the enable/disable regulator APIs.

Signed-off-by: Wesley Cheng <wcheng@codeaurora.org>
Link: https://lore.kernel.org/r/20200626185516.18018-6-wcheng@codeaurora.org
Signed-off-by: Mark Brown <broonie@kernel.org>