dt-bindings: gpio: pcf857x: Convert to json-schema

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: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Rob Herring <robh@kernel.org>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>