ARM: shmobile: r8a7779 dtsi: Add SoC-specific SATA compatible property

The SATA node used the generic compatible property only, which was
deprecated by commit e67adb4e ("sata_rcar: Add R-Car Gen2 SATA
PHY support"). Add the SoC-specific one introduced by that commit.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>