fdt: prevent clearing memory node if there are no banks

Avoid clearing the reg property in the memory DT node if no memory
banks have been specified for a board (CONFIG_NR_DRAM_BANKS == 0).
This allows boards to let U-Boot skip the DT memory tinkering in case
other firmware has already setup the node properly before.
This should be safe as all callers of fdt_fixup_memory_banks that use
a computed <banks> value put at least 1 in there.
Add some documentation comments to the header file.

Signed-off-by: Andre Przywara <osp@andrep.de>
Acked-by: Simon Glass <sjg@chromium.org>