dra7xx: Remove explicit DRAM banks number from defconfigs

dra7xx: Remove explicit DRAM banks number from defconfigs

The common EMIF init code used for DRA7xx does not explicitly fill
the gd->bd->bi_dram entries (like OMAP3 does), so there is no reason
to set an explicit number of DRAM banks which doesn't correspond to
anything in particular.

Remove the CONFIG_NR_DRAM_BANKS option from the concerned defconfigs.
The dram_init_banksize default implementation will be fine with the
default value for the config option.

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