Use newly added R-Car GPIO Gen1 fallback compat string
in place of now deprecated non-generation specific
R-Car GPIO fallback compat string in DT of r8a7778 SoC.

As the driver does not match on "renesas,gpio-r8a7778" there
are some run-time considerations for this patch:

* When a resulting DTB is used with kernels newer than v4.14 this should
  not have any run-time effect as renesas,rcar-gen1-gpio is matched by the
  driver since commit dbd1dad2 ("gpio: rcar: add gen[123] fallback
  compatibility strings")

* However, when used with older kernels GPIO will be disabled as
  no compat string match will be made by the driver.

The regression documented above for the new DTB with old kernel case
is acceptable in my opinion.
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>