x86 boot: add header comment to dmi.h stating what it is

The "dmi.h" file did not state anywhere in the file what "DMI" was.
For those who know, it's obvious.  For the rest of us, I added a
brief opening comment.

Signed-off-by: Paul Jackson <pj@sgi.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>