davinci: am18x/da850/omap-l138 evm: setup NAND flash timing

Setup the NAND flash timings for DA850 EVM

Before configuring the timing values, throughput calculation
using dd command yielded 469 kB/s write and 966 kB/s read speed.

After the timing configuration, the throughput was measured to
be 2.4 MB/s write and 5 MB/s read.

[Mukul Bhatnagar: actual calculation of timing values from the
NAND datasheet]

Signed-off-by: Sekhar Nori <nsekhar@ti.com>
Cc: Mukul Bhatnagar <mbhatnagar@ti.com>
Acked-by: Kevin Hilman <khilman@deeprootsystems.com>
Signed-off-by: Kevin Hilman <khilman@deeprootsystems.com>