Input: ads7846 - introduce platform specific way to synchronize sampling

Noises can be introduced when LCD signals are being driven, some platforms
provide a signal to assist the synchronization of this sampling procedure.

Signed-off-by: Eric Miao <eric.miao@marvell.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Dmitry Torokhov <dtor@mail.ru>