am33xx: Stop modifying certain EMIF4D registers

Based on the definitive guide to EMIF configuration[1] certain registers
that we have been modifying (and are documented registers) should be
left in their reset values rather than modified.  This has been tested
on AM335x GP EVM and Beaglebone White.

[1]: http://processors.wiki.ti.com/index.php/AM335x_EMIF_Configuration_tips


Cc: Enric Balletbo i Serra <eballetbo@iseebcn.com>
Cc: Javier Martinez Canillas <javier@dowhile0.org>
Cc: Heiko Schocher <hs@denx.de>
Cc: Lars Poeschel <poeschel@lemonage.de>
Signed-off-by: Tom Rini <trini@ti.com>
Tested-by: Matt Porter <matt.porter@linaro.org>