sheevaplug: correct SDRAM address control register

value

The SheevaPlug DevKit is shipped with 4x8 by 1Gb DDR devices in
two banks for a total of 512MB of RAM. Based on this configuration
the existing values for SDRAM address control register are incorrect
and result in random kernel oops as memory is incorrectly accessed
(while for example extracting a large tarball such as a rootfs).
Based on the hardware configuration along with the supporting
documentation from Marvell these are the correct values, as
well this change mimics values previously used in Marvell's own
u-boot git tree for the SheevaPlug.

Other variants of the hardware such as the PogoPlug and TonidoPlug
may have different memory configurations but to properly support
those additional board directories should be maintained or a better
system to support other kwb*.cfg is needed.

Tested on SheevaPlug DevKit.

Signed-off-by: Mark Asselstine <mark.asselstine@windriver.com>