mtd: add helpers to get the supportted ONFI timing mode

add onfi_get_async_timing_mode() to get the supportted asynchronous
timing mode.

add onfi_get_sync_timing_mode() to get the supportted synchronous
timing mode.

Also add the neccessary macros : the timing modes.

Signed-off-by: Huang Shijie <b32955@freescale.com>
Signed-off-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>