pinctrl: Enable "power-source" to be extracted from DT files

Add "power-source" property to generic options used for DT parsing files.
This  enables drivers, which use generic pin configurations, to get the
value passed to this property.

Signed-off-by: Ivan T. Ivanov <iivanov@mm-sol.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>