x86: Add documentation on the samus flashmap

x86: Add documentation on the samus flashmap

There are quite a few variables which control where things appear in the
final ROM image. Add a flashmap in the documentation to make this easier
to figure out.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
[bmeng: squashed "x86: Update the memory map a little" in]
[bmeng: fixed typo of 'documentation' in the commit title]
Signed-off-by: Bin Meng <bmeng.cn@gmail.com>