powerpc/8xxx: Replace fsl_ddr_get_mem_data_rate with get_ddr_freq()

Every 85xx board implements fsl_ddr_get_mem_data_rate via get_ddr_freq()
and every 86xx board uses get_bus_freq().  If implement get_ddr_freq()
as a static inline to call get_bus_freq() we can remove
fsl_ddr_get_mem_data_rate altogether and just call get_ddr_freq()
directly.

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