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: NBen Dooks <ben@simtec.co.uk>
Signed-off-by: NRob Landley <rob@landley.net>
Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>

Documentation/SM501.txt

@@ -3,6 +3,11 @@
 
 Copyright 2006, 2007 Simtec Electronics
 
+The Silicon Motion SM501 multimedia companion chip is a multifunction device
+which may provide numerous interfaces including USB host controller USB gadget,
+Asyncronous Serial ports, Audio functions and a dual display video interface.
+The device may be connected by PCI or local bus with varying functions enabled.
+
 Core
 ----