msm: qpnp-usbdetect: Make vin-supply optional

Some USB detection circuitries don't require an active supply
voltage, e.g. SMB135x or even a simple voltage divider. Thus allow
the vin-supply property to be optional.

Change-Id: I1fb01d3d8afb4029bc352de026012dfc2a8d688f
Signed-off-by: Jack Pham <jackp@codeaurora.org>