pinctrl: mvebu: make pdma clock on dove mandatory

With the ability to pass clocks through DT, now make the pdma
clock of dove pinctrl mandatory. Otherwise, pinctrl will hang
the system when accessing some registers.

Signed-off-by: Sebastian Hesselbarth <sebastian.hesselbarth@gmail.com>
Signed-off-by: Jason Cooper <jason@lakedaemon.net>