Tweak Documentation/SM501.txt

The existing Documentation/SM501.txt gives no clue what the chip is or does,
so copy the description from Kconfig help text.

Acked-by: Ben Dooks <ben@simtec.co.uk>
Signed-off-by: Rob Landley <rob@landley.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>