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/common.h

@@ -497,6 +497,7 @@ ulong	get_bus_freq  (ulong);
 #if defined(CONFIG_MPC85xx)
 typedef MPC85xx_SYS_INFO sys_info_t;
 void	get_sys_info  ( sys_info_t * );
+ulong	get_ddr_freq  (ulong);
 #endif
 #if defined(CONFIG_MPC86xx)
 typedef MPC86xx_SYS_INFO sys_info_t;