Input: bcm5974 - Preparatory renames

Rename touch properties to match established nomenclature, and define
the maximum number of fingers.

Tested-by: Benjamin Tissoires <benjamin.tissoires@enac.fr>
Acked-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>
Signed-off-by: Henrik Rydberg <rydberg@euromail.se>