arm64: dts: renesas: r8a7795: Add HDMI encoder support

Add DT nodes for the two HDMI encoders in disabled state.

Based on work by Koji Matsuoka.

Signed-off-by: Ulrich Hecht <ulrich.hecht+renesas@gmail.com>
Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>