clk: shmobile: rcar-gen3: Add CPG/MSTP Clock Domain support

Add Clock Domain support to the R-Car Gen3 Clock Pulse Generator (CPG)
driver using the generic PM Domain.  This allows to power-manage the
module clocks of SoC devices that are part of the CPG/MSTP Clock Domain
using Runtime PM, or for system suspend/resume.

SoC devices that are part of the CPG/MSTP Clock Domain and can be
power-managed through an MSTP clock should be tagged in DT with a proper
"power-domains" property.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Magnus Damm <damm+renesas@opensource.se>
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>