ARM: davinci: dm646x: Fix a typo in the comment

The driver is dedicated to DM646x. So update the description in the top
most comment accordingly.

It must have been derived from dm644x.c, but looks DM646 speecific now.

Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Signed-off-by: Sekhar Nori <nsekhar@ti.com>