[media] cec: add STM32 cec driver

This patch add cec driver for STM32 platforms.
cec hardware block isn't not always used with hdmi so
cec notifier is not implemented. That will be done later
when STM32 DSI driver will be available.

Driver compliance has been tested with cec-ctl and cec-compliance
tools.

Signed-off-by: Benjamin Gaignard <benjamin.gaignard@linaro.org>
Signed-off-by: Yannick Fertre <yannick.fertre@st.com>
[hans.verkuil@cisco.com: modified platform/Makefile to use obj-y]
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>

Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>