TI: armv7: Do not define the number DRAM banks if is already defined.

If CONFIG_NR_DRAM_BANKS is not defined, we say (for simplicity) that we have
1 bank, but for some boards should be interesting that we can define
CONFIG_NR_DRAM_BANKS. To handle this possibility just define the number of
DRAM banks if is not already defined. This is useful for some OMAP3 boards
where the DRAM initialitzation is only at u-boot level.

Signed-off-by: Enric Balletbo i Serra <eballetbo@gmail.com>