arm64: dts: r8a7796: Use R-Car GPIO Gen3 fallback compat string

Use newly added R-Car GPIO Gen3 fallback compat string
in place of now deprecated non-generation specific
R-Car GPIO fallback compat string in the DT of the r8a7796 SoC.

This should have no run-time effect as the driver matches against
the per-SoC compat string before considering the fallback compat string.

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