Documentation: dt: update TI GPMC ethernet binding properties

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 ethernet binding.

Signed-off-by: Javier Martinez Canillas <javier.martinez@collabora.co.uk>
Signed-off-by: Jon Hunter <jon-hunter@ti.com>