USB: msm_otg: Make power supply properties writeable

Define a property_is_writeable() power supply framework callback
function for the USB power supply device which specifies that the
power supply properties: present, online, and current_max are
writeable from userspace.

Adding write permissions for these properties in sysfs enables
further debug and testing capabilities for the msm_otg driver.
One particularly useful case is the ability to force a connected
charger power supply device into USB suspend mode without any
involvement from the USB host.

Change-Id: Iddd3d350fbf561d174551566d1ddc3848c896566
Signed-off-by: David Collins <collinsd@codeaurora.org>