configs: rpi: set NR_DRAM_BANKS to 8 to accommodate RAM on 16GB models

configs: rpi: set NR_DRAM_BANKS to 8 to accommodate RAM on 16GB models

Raspberry Pi 5 can now have up to 16 GiB of RAM where the memory spans 8
DRAM banks in total. Increase the config value to 8 to initialize the
whole RAM. Without this change, kernel only sees 8 GiB of RAM on the 16
GiB CM5 as reported in [1].

[1] https://github.com/home-assistant/operating-system/issues/3989

Signed-off-by: Jan Čermák <sairon@sairon.cz>
Reviewed-by: Matthias Brugger <mbrugger@suse.com>