pinctrl: mvebu: armada-39x: normalize ref clock naming

This commit normalizes the subnames of the reference clock MPP pins in
the Armada 39x pinctrl driver to match with the name used on other
SoCs.

Since only the subnames are changed, DT backward compatibility is not
affected.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>