usb: gadget: Add snapshot of USB GSI function driver

This change adds USB GSI function driver which provides USB
RMNET/RNDIS/ECM/MBIM/DPL related functionalities with GSI hardware
accelerated path.

This snapshot is taken as of msm-4.9 commit 4f7918fd623f ("usb: f_gsi:
Use required RNDIS IAD descriptors with full speed descriptors").

Registering a misc device is discouraged, particularly when
MISC_DYNAMIC_MINOR is used as there are a limited (64) number
of dynamic minors available and can quickly exhaust. Convert
f_gsi's usage to struct cdev instead.

Change-Id: Ia6915c76deb084ebb789ae33c509e835502cfcf1
Signed-off-by: Jack Pham <jackp@codeaurora.org>