ARM: dts: r8a7791: Convert to new CPG/MSSR bindings

Convert the R-Car M2-W SoC from the old "Renesas R-Car Gen2 Clock Pulse
Generator (CPG)", "Renesas CPG DIV6 Clock", and "Renesas CPG Module Stop
(MSTP) Clocks" DT bindings to the new unified "Renesas Clock Pulse
Generator / Module Standby and Software Reset" DT bindings.

This simplifies the DTS files, and allows to add support for reset
control later.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>