usb: gadget: Allow non-default and non-zero ITC value

Allow setting ITC value provided from device tree. In case no value
provided from device tree, check the CI13XXX_ZERO_ITC flag, and
if set use the value of 0 as the ITC.

Change-Id: I9333124913b7ae737eabc8eb5d154d99ac184ee8
Signed-off-by: Amit Blay <ablay@codeaurora.org>