dt-bindings: clock: renesas: div6: Convert to json-schema

Convert the Renesas CPG DIV6 Clock Device Tree binding documentation to
json-schema.

Drop R-Car Gen2 compatible values, which were obsoleted by the unified
"Renesas Clock Pulse Generator / Module Standby and Software Reset" DT
bindings.
Update the example to match reality.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Stephen Boyd <sboyd@kernel.org>
Reviewed-by: Rob Herring <robh@kernel.org>
Reviewed-by: Niklas Söderlund <niklas.soderlund+renesas@ragnatech.se>
Link: https://lore.kernel.org/r/20200507075026.31941-1-geert+renesas@glider.be