Some of the GPMC timings parameters are currently missing from the GPMC
device-tree binding. Add these parameters to the binding documentation
as well as code to read them. Also add either "-ps" or "-ns" suffix to
the GPMC timing properties to indicate whether the timing is in
picoseconds or nanoseconds.

The existing code in gpmc_read_timings_dt() is checking the value of
of_property_read_u32() and only is successful storing the value read
in the gpmc_timings structure. Checking the return value in this case
is not necessary and we can simply read the value, if present, and
store directly in the gpmc_timings structure. Therefore, simplify the
code by removing these checks.

The comment in the gpmc_read_timings_dt() function, "only for OMAP3430"
is also incorrect as it is applicable to all OMAP3+ devices. So correct
this too.

Signed-off-by: Jon Hunter <jon-hunter@ti.com>
Tested-by: Ezequiel Garcia <ezequiel.garcia@free-electrons.com>