pinctrl: enhance pinctrl_get() to handle multiple functions

At present, pinctrl_get() assumes that all matching mapping table entries
have the same "function" value, albeit potentially applied to different
pins/groups.

This change removes this restriction; pinctrl_get() can now handle a set
of mapping tables where different functions are applied to the various
pins/groups.

Signed-off-by: Stephen Warren <swarren@nvidia.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>