[media] cec: s5p-cec: Add s5p-cec driver

Add CEC interface driver present in the Samsung Exynos range of
SoCs.

The following files were based on work by SangPil Moon:
- exynos_hdmi_cec.h
- exynos_hdmi_cecctl.c

Signed-off-by: Kamil Debski <kamil@wypas.org>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>