Input: matrix-keymap - wire up device tree support

When platform keymap is not supplied to matrix_keypad_build_keymap()
and device tree support is enabled, try locating specified property
and load keymap from it. If property name is not defined, try using
"linux,keymap".

Based on earlier patch by Viresh Kumar <viresh.kumar@st.com>

Signed-off-by: Dmitry Torokhov <dtor@mail.ru>