[PATCH v3] Add sync to ensure flash_write_cmd is fully finished

Some CPUs like PPC, BLACKFIN need sync() to ensure cfi flash write command
is fully finished. The sync() is defined in each CPU's io.h file. For
those CPUs which do not need sync for now, a dummy sync() is defined in
their io.h as well.

Signed-off-by: Haiying Wang <Haiying.Wang@freescale.com>