pinctrl: berlin: prepare to use regmap provided by syscon

The Berlin pin controller nodes will be simple-mfd probed sub-nodes of
soc-controller and system-controller nodes. The register bank is managed
by syscon, which provides a regmap.

Prepare to get the regmap from syscon parent node instead of SoC stub
provided regmap.

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