fs: fatwrite: use map_sysmem before use file_fat_write

When the map_sysmem, then the fatwrite command can support sandbox.
Following command will show how to use it:
  => sb bind 0 sd.img
  => fatls host 0
  => fatwrite host 0 $memaddr filename $filesize

Signed-off-by: Josh Wu <josh.wu@atmel.com>
Acked-by: Simon Glass <sjg@chromium.org>