usb: gadget: Add Android Composite Gadget driver

The Android Gadget driver is a composite driver that allows
userspace to change at runtime the list of functions enabled in
its configuration and to configure these functions. It supports
multiple functions: acm, adb, rndis, mtp/ptp, mass storage and
accessory.

It is usually controlled by a daemon that changes the configuration
based on user settings. For example, rndis is enabled when the user
enables sharing the phone data connection and adb (Android Debug
Bridge) is only enabled when the user wants to debug applications
for security reasons.

As an example on how to use it, the following shell commands will
make the gadget disconnect from the host and make it be re-enumerated
as a composite with 1 rndis and 2 acm interfaces, and a different
product id:

echo 0       > /sys/class/android_usb/android0/enable
echo rndis,acm > /sys/class/android_usb/android0/functions
echo 2       > /sys/class/android_usb/android0/f_acm/instances
echo 2d01    > /sys/class/android_usb/android0/idProduct
echo 1       > /sys/class/android_usb/android0/enable

The driver requires a gadget controller that supports software
control of the D+ pullup and the controller driver must support
disabling the pullup during composite_bind.

Signed-off-by: Mike Lockwood <lockwood@android.com>
Signed-off-by: Benoit Goby <benoit@android.com>