Input: add st-keyscan driver

This patch adds ST Keyscan driver to use the keypad hw a subset of ST
boards provide. Specific board setup will be put in the given dt.

Signed-off-by: Gabriel Fernandez <gabriel.fernandez@linaro.org>
Signed-off-by: Giuseppe Condorelli <giuseppe.condorelli@st.com>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>