ARM: shmobile: rcar-gen2: Drop legacy DT clock support

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 backward compatibility with old R-Car Gen2 device trees
describing a hierarchical representation of the various CPG and MSTP
clocks.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Link: https://lore.kernel.org/r/20191016150939.30620-1-geert+renesas@glider.be