pinctrl: berlin: add the BG2 pinctrl driver

Add the pin-controller driver for the Berlin BG2 SoC, with definition
of its groups and functions. Pin control registers are part of chip/
system control registers, which will be represented by a single node.
Until a proper driver for the chip/system control is available,
register the corresponding regmap in pinctrl driver probe.

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