ARM: imx_v6_v7_defconfig: enable USB FFS gadget

ARM: imx_v6_v7_defconfig: enable USB FFS gadget

The USB Function FS gadget can be very useful for debugging.

For instance, the adbd daemon can leverage this gadget to
offer an ADB connection to the platform over USB.

Note that adbd is available on many OS/build systems:
- Ubuntu: see android-tools-adbd package [1]
- Yocto: see meta-smartphone android-tools recipe [2]
- Buildroot: see android-tools package [3]

[1] http://packages.ubuntu.com/trusty/android-tools-adbd
[2] https://github.com/shr-distribution/meta-smartphone/tree/shr/meta-android/recipes-android/android-tools
[3] https://git.buildroot.net/buildroot/tree/package/android-tools

Signed-off-by: Gary Bisson <gary.bisson@boundarydevices.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>