Convert the Renesas DDR Bus Controller Device Tree binding documentation
to json-schema.

Drop referrals to driver behavior.
Make power-domains required, as it is present for all current users.
Update the example to match reality.
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Rob Herring <robh@kernel.org>