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>