input: convert the kscan_sdl driver from kscan to input

Convert the SDL driver to use the input subsystem. This is specifically
meant to emulate touchscreen drivers, so it's setup to send triplet of
x, y, touch for touch-on events and just touch off on touch off events.

Renamed the driver to input-sdl-touch since now we can also develop an
sdl driver for simulating key events.

Signed-off-by: Fabio Baltieri <fabiobaltieri@google.com>