usb: msm_otg: Add support for voltage_max power supply property

Allow voltage_max property to be written from user space.  The USB
power supply input voltage can be limited by writing into
/sys/class/power_supply/usb/voltage_max sysfs file.  The user space
charger detection application uses this property value to place
an appropriate voltage request to the charger.

Change-Id: Id997a97dee073d0d8f877b58470ca2f70e89065e
Signed-off-by: Pavankumar Kondeti <pkondeti@codeaurora.org>