ppc/p4080: Fix synchronous frequency calculations

When DDR is in synchronous mode, the existing code assigns sysclk
frequency to DDR frequency.  It should be synchronous with the platform
frequency.  CPU frequency is based on platform frequency in synchronous
mode.

Also fix:

* Fixes the bit mask for DDR_SYNC (RCWSR5[184])
* Corrects the detection of synchronous mode.

Signed-off-by: Srikanth Srinivasan <srikanth.srinivasan@freescale.com>
Signed-off-by: Dave Liu <daveliu@freescale.com>
Signed-off-by: Ed Swarthout <Ed.Swarthout@freescale.com>
Signed-off-by: Kumar Gala <galak@kernel.crashing.org>