Use newly added R-Car SATA Gen2 fallback compat string
in the DT of the r8a7791 SoC.

This should have no run-time effect as the driver matches against
the per-SoC compat string before the fallback compat string is considered.
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>