drm: renesas: shmobile: Add DT support

Add DT support, by:
  1. Creating a panel bridge from DT, and attaching it to the encoder,
  2. Replacing the custom connector with a bridge connector,
  3. Obtaining clock configuration based on the compatible value.

Note that for now the driver uses a fixed clock configuration selecting
the bus clock, as the current code to select other clock inputs needs
changes to support any other SoCs than SH7724.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Link: https://lore.kernel.org/r/6185ab76aa300fa402e4f6610b2109665f2d8a1c.1694767209.git.geert+renesas@glider.be