fdt_support: Fix buffer overflow in fdt_fixup_memory_banks

When fdt_fixup_memory_banks is called with 2-cell address and size
fields in the device-tree (IE: 64-bit address and size), then it will
overflow its on-stack "tmp" buffer.

This fixes the buffer size and adds a comment explaining how many bytes
need to be allocated per record.

Signed-off-by: Kyle Moffett <Kyle.D.Moffett@boeing.com>
Cc: Jerry Van Baren <vanbaren@cideas.com>
Acked-by: Gerald Van Baren <vanbaren@cideas.com>