Input: add support for SuperH MigoR touchscreen

This is V2 of the MigoR touch screen driver. The chip we interface to
is unfortunately a custom designed microcontroller speaking some
undocumented protocol over i2c.

The board specific code is expected to register this device as an i2c
chip using struct i2c_board_info [] and i2c_register_board_info().

[dtor@mail.ru: don't enable touchscreen if there are no users]
Signed-off-by: Magnus Damm <damm@igel.co.jp>
Signed-off-by: Dmitry Torokhov <dtor@mail.ru>