Documentation: dt: update properties in TI GPMC NAND example

The GPMC timing properties for device-tree have been updated
by adding a "-ns" or "-ps" suffix to indicate the units of
time the property represents. Therefore, update the timing
property names for TI GPMC NAND example.

Signed-off-by: Jon Hunter <jon-hunter@ti.com>
Acked-by: Grant Likely <grant.likely@secretlab.ca>