OMAP3EVM: Added NAND support

The EVMS have been shipping with NAND (instead of OneNAND) as default.
So, this patch sets NAND as default.

To choose OneNAND, define CMD_ONENAND instead of CMD_NAND in the
config file omap3_evm.h.

Signed-off-by: Vaibhav Hiremath <hvaibhav@ti.com>
Signed-off-by: Sandeep Paulraj <s-paulraj@ti.com>