Add R-Car V3M (R8A77970) support to the DU driver

Hello!

Here's the set of 2 patches against the 'drm-next' branch of David Airlie's 'linux.git' repo. The purpose of these patches is to add the R-Car V3M (R8A77970) support to the DU driver; the LVDS driver patches will be posted separately...

[1/2] DT: display: renesas,du: document R8A77970 bindings [2/2] drm: rcar-du: add R8A77970 support

MBR, Sergei