Document support for the GPIO controller blocks in the Renesas R-Car
S4-8 (R8A779F0) SoC, including a new family-specific compatible value
for the R-Car Gen4 family.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Signed-off-by: Bartosz Golaszewski <brgl@bgdev.pl>