sata_rcar: Document deprecated "renesas,rcar-sata"

Commit e67adb4e ("sata_rcar: Add R-Car Gen2 SATA PHY support")
deprecated "renesas,rcar-sata" in favor of "renesas,sata-r8a7779", but
the deprecated value was never documented in the binding documentation,
while it is still in active use.

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>