fdt_support: Use CONFIG_NR_DRAM_BANKS if defined

It appears that there are some cases where we have more than 4 banks
of memory.  Use CONFIG_NR_DRAM_BANKS if it's defined to handle this.
This will take up a little extra stack space (64 bytes extra if we go
up to 8 banks), but that seems OK.

Signed-off-by: Doug Anderson <dianders@chromium.org>