rpi: Enable DRAM bank initialization on arm64

Up to now we only update the DRAM banks when we are define
CONFIG_BCM2711. But our one binary approach uses a config that supports
BCM2837 and BCM2711. As a result we only see one gibibyte of RAM on
Raspberry Pi 4, even if it has more RAM.
Fix this by calling dram_init_banksize.

Fixes: 56940906 ("ARM: defconfig: add unified config for RPi3 and RPi4")

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