pinctrl: samsung: Distinguish between pin group and bank nodes

This patch modifies the loop iterating over all child nodes and parsing
pin groups to check whether the node is really a pin group node by
checking for existence of samsung,pins property.

This is a prerequisite for further patches adding additional subnodes to
the pinctrl node, required for per bank GPIO and interrupt specifiers.

Signed-off-by: Tomasz Figa <t.figa@samsung.com>
Reviewed-by: Kyungmin Park <kyungmin.park@samsung.com>
Acked-by: Thomas Abraham <thomas.abraham@linaro.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>