Add a brief document about omap2430/3430 HDQ/1-wire driver.

Signed-off-by: Madhusudhan Chikkature<madhu.cr@ti.com>
Acked-by: NEvgeniy Polyakov <johnpol@2ka.mipt.ru>
Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>