am335x: ddr_defs: Update EMIF parameters

am335x: ddr_defs: Update EMIF parameters

EMIF parameters are calculated based on the AC timing
parameters from the SDRAM datasheet and the DDR frequency.

Current values for these paramters in AM335x U-Boot code,
though reliable, are not fully optimal. The most optimal
settings can be derived based on the guidelines published
at [1]. A pre-computed set of values with the most optimum
settings for AM335x EVM and BeagleBone can be found at [2].

[1] http://processors.wiki.ti.com/index.php/AM335x_EMIF_Configuration_tips
[2] http://processors.wiki.ti.com/index.php/OMAP_and_Sitara_CCS_support#AM335x

Signed-off-by: Vaibhav Bedia <vaibhav.bedia@ti.com>
Signed-off-by: Tom Rini <trini@ti.com>