clk: renesas: rcar-gen3: Restore R clock during resume

On R-Car Gen3 systems, PSCI system suspend powers down the SoC, losing
clock configuration.  Register a notifier to save/restore the RCKCR
register during system suspend/resume.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Tested-by: Niklas Söderlund <niklas.soderlund+renesas@ragnatech.se>