dt-bindings: clock: renesas: Remove R-Car Gen2 legacy DT bindings

As of commit 362b334b ("ARM: dts: r8a7791: Convert to new
CPG/MSSR bindings"), all upstream R-Car Gen2 device tree source files
use the unified "Renesas Clock Pulse Generator / Module Standby and
Software Reset" DT bindings.

Hence remove the old R-Car Gen2 DT bindings describing a hierarchical
representation of the various CPG and MSTP clocks.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Acked-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20191016145207.29779-1-geert+renesas@glider.be