SM501: Add Documentation/SM501.txt

Add documentation for the SM501 in Documentation/SM501.txt outlining the SM501
driver.

Signed-off-by: Ben Dooks <ben-linux@fluff.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>