drm: rcar-du: Add Gen3 HDMI encoder support

The R-Car Gen3 SoCs include on-chip DesignWare HDMI encoders. Support
them with a platform driver to provide platform glue data to the dw-hdmi
driver.

The driver is a complete rewrite of code coming from the Renesas BSP,
save for the values in the PHY parameters table.

Signed-off-by: Koji Matsuoka <koji.matsuoka.xm@renesas.com>
Signed-off-by: Ulrich Hecht <ulrich.hecht+renesas@gmail.com>
Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Signed-off-by: Kieran Bingham <kieran.bingham+renesas@ideasonboard.com>