[POWERPC] 83xx: mpc8315 - fix USB UTMI Host setup

Currently USB Host isn't functional on the MPC8315E boards, for two
reasons as described below.

MPC8315 Reference Manual says:
"The USB DR unit must have the same clock ratio as the encryption core
unit, unless one of them has its clock disabled."

The encryption core also drives I2C clock, so it is enabled and is equal
to 01. That means USBDRCM should be 01 here.

Plus, according to MPC8315E-RDB schematics, USB unit consumes CLK_IN
clock from the 24.00MHz oscillator, which means we must adjust REFSEL
bits as well.

p.s.
Idially we should rework whole 83xx/usb.c code, in two steps:
1. Move SCCR code to the U-Boot;
2. Implement fsl,usb-clock property in the device tree, so usb.c could
   decide what clock exactly to use on per-board basis.

Though, today we're not in a hurry since there is just one 8315e board
out there.

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