[POWERPC] 83xx: USB platform code rework

Add 831x USB platform setup code and rework 834x USB platform setup code.
Move USB platform code to usb.c for different boards with CPU of the same
series to share the USB initialization code.

Signed-off-by: Li Yang <leoli@freescale.com>
Signed-off-by: Kim Phillips <kim.phillips@freescale.com>
Signed-off-by: Kumar Gala <galak@kernel.crashing.org>