msm: ultrasound: multi input devices support.

User space based UI framework enables additional configuration
of input devices  along with configuration in the proper kernel drivers.
To use this capability in the ultrasound framework,
the USF driver should simulate multiple input devices:
one device for each input event type.

Change-Id: Idd6aad0814ea57796ab789758fd28ae46d6e9d8a
Signed-off-by: Baruch Eruchimovitch <baruche@codeaurora.org>