ARM: STi: DT: add keyscan for stih415

Add keyscan support for stih415.
It is put disabled by default because it is not enabled on all boards
Also there are PIOs conflict with already claimed lines.

Acked-by: Srinivas Kandagatla <srinivas.kandagatla@linaro.org>
Signed-off-by: Giuseppe Condorelli <giuseppe.condorelli@st.com>
Signed-off-by: Gabriel Fernandez <gabriel.fernandez@linaro.org>
Signed-off-by: Maxime Coquelin <maxime.coquelin@st.com>