Handle Asynchronous DDR clock on 85xx

The MPC8572 introduces the concept of an asynchronous DDR clock with
regards to the platform clock.

Introduce get_ddr_freq() to report the DDR freq regardless of sync/async
mode.

Signed-off-by: Kumar Gala <galak@kernel.crashing.org>

include/e500.h

@@ -12,6 +12,7 @@ typedef struct
 {
   unsigned long freqProcessor;
   unsigned long freqSystemBus;
+  unsigned long freqDDRBus;
 } MPC85xx_SYS_INFO;
 
 #endif  /* _ASMLANGUAGE */