ARM: defconfig: Fix 32bit config for RPi4

The rpi_4_32b_defconfig states that only one DRAM bank is present. This
leads to a wrong configuration of the available DRAM. Fix this by
setting the DRAM bank config accordingly.

Fixes: 193279d7 ("RPI: Add defconfigs for rpi4 (32/64)")

Signed-off-by: Matthias Brugger <mbrugger@suse.com>