Flush cache after the OS image is loaded into the memory.

Since we are loading an executable image into memory we need flush it
out of the cache to possible maintain coherence on CPUs with split
instruction and data caches.  We do this for other executable image
loading command.

On PowerPC once we do this we no longer need to explicitly flush the
dcache on multi-core systems in the BOOTM_STATE_OS_PREP phase.  We now
treat the BOOTM_STATE_OS_PREP as a no-op to maintain backwards
compatibility with the bootm subcommand.

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