Input: introduce EV_MSC Timestamp

Some devices provides the actual timestamp (hid_dg_scan_time in win8 ones)
computed by the hardware itself. This value is global to the frame and is
not specific to the multitouch protocol.

Signed-off-by: Benjamin Tissoires <benjamin.tissoires@gmail.com>
Reviewed-by: Henrik Rydberg <rydberg@euromail.se>
Acked-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>