rpi: fix dram bank initialization

To update the dram bank information from device-tree we use
fdtdec_decode_ram_size() which expectes the the size-cells and
address-cells to be defined in the memory node. For normal system RAM
these values are defined in the root node. When the values differ from
the default values defined in the spec, we can end up with wrong RAM
bank information.

Switch to the "standard" way to update the RAM bank information to
avoid this.

Fixes: 9de5b89e ("rpi4: enable dram bank initialization")

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