pinctrl: bcm: ns: support updated DT binding as syscon subnode

Documentation has been recently updated specifying that pinctrl should
be subnode of the CRU "syscon". Support that by using parent node for
regmap and reading "offset" property from the DT.

Signed-off-by: Rafał Miłecki <rafal@milecki.pl>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>