85xx: Speed up get_ddr_freq() and get_bus_freq()

get_ddr_freq() and get_bus_freq() used get_sys_info() each time they were
called.  However, get_sys_info() recalculates extraneous information when
called each time.  Have get_ddr_freq() and get_bus_freq() return memoized
values from global_data instead.

Signed-off-by: James Yang <James.Yang@freescale.com>
Signed-off-by: Kumar Gala <galak@kernel.crashing.org>