Convert the PCF857x-compatible I/O expanders Device Tree binding
documentation to json-schema.

Document missing compatible values, properties, and gpio hogs.
Signed-off-by: NGeert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: NRob Herring <robh@kernel.org>
Reviewed-by: NLinus Walleij <linus.walleij@linaro.org>
Signed-off-by: NBartosz Golaszewski <bgolaszewski@baylibre.com>