pinctrl: dt-binding: document berlin4ct SoC pinctrl

Add berlin4ct to existing berlin pinctrl device tree binding.

Signed-off-by: Jisheng Zhang <jszhang@marvell.com>
Acked-by: Sebastian Hesselbarth <sebastian.hesselbarth@gmail.com>
Acked-by: Antoine Tenart <antoine.tenart@free-electrons.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>