ARM: STi: STiH416-b2020e: Enable PWM on the B2020 Rev-E

All the infrastructure is now in place for ST's PWM controller.  This
patch takes the final step and enables the IP on the 2020 Rev-E
development platform.

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>