rpi4: enable dram bank initialization

When booting through the efi stub, the memory map get's created by
reading the dram bank information. Depending on the version of the RPi4
this information changes. Read the device tree to initialize the dram
bank data structure. This way the kernel is able to access the whole
range of available memory.

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