arm64: dts: r8a7795: Use Gen 3 fallback compat string for PCIE

Use recently added en 3 fallback compat string for PCIE
in r8a7795 DT.

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