USB: msm_otg: Implement VOLTAGE_NOW property for usb power supply

Implement VOLTAGE_NOW property for power supply so that usb_in
value can be read at any time through this sysfs entry by
userspace.

To read usb_in value,
	cat /sys/class/power_supply/usb/voltage_now

CRs-Fixed: 599547
Change-Id: I8f9bf4077282fbe32304b667793dab8009e68243
Signed-off-by: Vijayavardhan Vennapusa <vvreddy@codeaurora.org>