dts: rcar: Rework clock definition

This commit is reworking clock definition in
order to match linux filetree and definition
as much as possible.

- dt-bindings: rework renesas clocks dt-bindings
  - regroup renesas related dt-bindings in a folder
  - rename renesas rcar common dt-binding to match linux name
  - add soc specific dt-binding matching linux name
  - soc dt-bindings are defining clocks matching linux names

- dts: use new clocks names
  - move clocks definitions in SoC layer for each core clock entry

- driver: use new clocks names

As seen in this commit, we are declaring clocks for "R8A7795" SoC
to match linux names.
Linux is not declaring "R8A77951" SoC specific files because
its also supporting the first H3 SoC version numbered "R8A77950".

Signed-off-by: Aymeric Aillet <aymeric.aillet@iot.bzh>