drm/exynos: Add DECON driver

drm/exynos: Add DECON driver

This patch is based on exynos-drm-next branch of Inki Dae's tree at:
git://git.kernel.org/pub/scm/linux/kernel/git/daeinki/drm-exynos.git

DECON(Display and Enhancement Controller) is the new IP
in exynos7 SOC for generating video signals using pixel data.

DECON driver can be used to drive 2 different interfaces on Exynos7:
DECON-INT(video controller) and DECON-EXT(Mixer for HDMI)

The existing FIMD driver code was used as a template to create
DECON driver. Only DECON-INT is supported as of now, and
DECON-EXT support will be added later.

The current version of the driver supports video mode displays.

Changelog v2:
- Change config name, DRM_EXYNOS_DECON to DRM_EXYNOS7_DECON.

Signed-off-by: Akshu Agrawal <akshua@gmail.com>
Signed-off-by: Ajay Kumar <ajaykumar.rs@samsung.com>
Signed-off-by: Inki Dae <inki.dae@samsung.com>