powerpc/82xx: Add CPM USB Gadget support for MPC8272ADS boards

- Add usb node;
- Configure pins and clocks;
- Enable USB function in BCSR.

The support was successfully tested using serial and ethernet gadget
drivers.

Signed-off-by: Anton Vorontsov <avorontsov@ru.mvista.com>
Signed-off-by: Kumar Gala <galak@kernel.crashing.org>