sata_rcar: Add clocks to sata_rcar bindings

Now that the clocks are available in the R-Car Gen2 DT,
add clocks property description to the sata_rcar bindings.
The clocks have been tested on r8a7791 so we use that
as an example of the R-Car SATA node.

Signed-off-by: Valentine Barshak <valentine.barshak@cogentembedded.com>
[geert: Reworded clocks property]
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Acked-by: Simon Horman <horms+renesas@verge.net.au>
Signed-off-by: Tejun Heo <tj@kernel.org>