configs: am62x_evm_r5: Add CONFIG_NR_DRAM_BANKS as done in a53 defconfig

Add CONFIG_NR_DRAM_BANKS from am62x_evm_a53_defconfig as this is
needed to calculate the size of DDR that is available.

Signed-off-by: Georgi Vlaev <g-vlaev@ti.com>
Reviewed-by: Tom Rini <trini@konsulko.com>