drm/exynos: exynos7_drm_decon: add driver data and support for Exynos7870

Add support for Exynos 7870 DECON in the Exynos 7 DECON driver.

Some Exynos 7 series SoCs (Exynos 7580 onwards) have different
register values. In order to address such changes, include a driver
data struct (named decon_data) so that correct base addresses and
shift values can be provided.

Signed-off-by: Kaustabh Chakraborty <kauschluss@disroot.org>
Signed-off-by: Inki Dae <inki.dae@samsung.com>