davinci: omapl138_lcdk: fix tXSNR DDR2 timing value

As per the datasheet[1] available for DDR2 part on board
the OMAP-L138 LCDK, the tXSNR (exit self refresh to a
non-read command) is 137.5 ns. This corresponds to a
value of 20 to be written to T_XSNR register field of
OMAP-L138's DDR configuration. The DDR2 is at 150 MHz.

Fix this. The correct value also appears on the initialization
scripts (called CCS GEL files) available on TI's wiki pages[2]

[1] http://www.samsung.com/global/business/semiconductor/file/product/ds_k4t1gxx4qf_rev12-0.pdf
[2] http://processors.wiki.ti.com/index.php/L138/C6748_Development_Kit_(LCDK)#CCS_XML_.26_GEL_Files

Reviewed-by: Tom Rini <trini@konsulko.com>
Signed-off-by: Sekhar Nori <nsekhar@ti.com>