pinctrl: dove: add PMU functions to pinctrl

Dove power management unit can mux some special functions to mpp0-15.
This patch adds support to set/get the current PMU function mapped
to the corresponding mpp pins. The device tree documentation is also
updated accordingly.

Signed-off-by: Sebastian Hesselbarth <sebastian.hesselbarth@gmail.com>
Acked-by: Jason Cooper <jason@lakedaemon.net>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>