omap3: Define DRAM banks number in Kconfig instead of defconfigs

omap3: Define DRAM banks number in Kconfig instead of defconfigs

The number of DRAM banks was defined to the same value in each OMAP3
board defconfig, which is expected and hardcoded in the code. Move the
common definition to the Kconfig option declaration instead.

Signed-off-by: Paul Kocialkowski <contact@paulk.fr>
Reviewed-by: Tom Rini <trini@konsulko.com>
Tested-by: Derald D. Woods <woods.technical@gmail.com>