pinctrl: mvebu: fix MPP6 value for kirkwood driver

Note that I am not sure about the MPP value for the PTP functionality.
It seems that the PTP references have been removed from the Marvell
hardware specifications available to me.

Signed-off-by: Simon Guinot <simon.guinot@sequanux.org>
Acked-by: Jason Cooper <jason@lakedaemon.net>
Acked-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>