ata: sata_rcar: Add rudimentary Runtime PM support

Replace the explicit clock handling to enable/disable the SATA module by
calls to Runtime PM.

This makes the driver independent of actual SoC clock/power hierarchies,
and is needed to support virtualization, where the guest is not in full
control of power management.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Sergei Shtylyov <sergei.shtylyov@cogentembedded.com>
Signed-off-by: Tejun Heo <tj@kernel.org>