[POWERPC] 83xx: configure USB clock for MPC8315E

SCCR USB bits are in a different location on the mpc8315.

Signed-off-by: Jerry Huang <Chang-Ming.Huang@freescale.com>
Signed-off-by: Kim Phillips <kim.phillips@freescale.com>
Signed-off-by: Kumar Gala <galak@kernel.crashing.org>