fdt_support: Add kernel-doc for fdt_fixup_memory_banks()

Add kernel-doc description for fdt_fixup_memory_banks() because it is
implemented in one specific way and this information should be available
for others to decide if their SoC conforms to it.
If you don't want U-Boot to update your memory DT layout please disable
CONFIG_ARCH_FIXUP_FDT_MEMORY.

Signed-off-by: Michal Simek <michal.simek@xilinx.com>
Reviewed-by: Simon Glass <sjg@chromium.org>