mpc8xxx: LCRR[CLKDIV] is sometimes five bits

On newer CPUs, 8536, 8572, and 8610, the CLKDIV field of LCRR is five bits
instead of four.

In order to avoid an ifdef, LCRR_CLKDIV is set to 0x1f on all systems.  It
should be safe as the fifth bit was defined as reserved and set to 0.

Code that was using a hard coded 0x0f is changed to use LCRR_CLKDIV.

Signed-off-by: Trent Piepho <tpiepho@freescale.com>
Acked-by: Kumar Gala <galak@kernel.crashing.org>
Acked-by: Jon Loeliger <jdl@freescale.com>