spi: dt-bindings: rspi: Convert to json-schema

Convert the Renesas (Quad) Serial Peripheral Interface (RSPI/QSPI)
Device Tree binding documentation to json-schema.

Document missing properties.
Update the second example to match reality.
Drop the first example, as it doesn't add much value.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20200408091129.25429-1-geert+renesas@glider.be
Signed-off-by: Mark Brown <broonie@kernel.org>