[PATCH] I2C: add adm9240 driver documentation

This patch adds adm9240 driver doc, with thanks to Rudolf Marek
for review.

Signed-off-by: Grant Coady <gcoady@gmail.com>
Acked-by: Rudolf Marek <r.marek@sh.cvut.cz>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>