pinctrl: samsung: Allow per-bank SoC-specific private data

This patch extends pin bank descriptor structure with SoC-specific
private data field that allows SoC-specific drivers to store their own
private data.

Signed-off-by: Tomasz Figa <t.figa@samsung.com>
Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>
Tested-by: Doug Anderson <dianders@chromium.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>