ARM: STi: STiH407: Add DT nodes for for PWM

Supply top level nodes for the STiH407 based development boards.
The Pinctrl configuration has already been applied, so the only
missing piece of the DT puzzle is for a board's DTB to enable
the nodes.

Signed-off-by: Ajit Pal Singh <ajitpal.singh@st.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Maxime Coquelin <maxime.coquelin@st.com>