pinctrl: mvebu: armada-375: remove non-existing NAND re/we pins

After updating to a more recent version of the Armada 375, we realized
that some of the pins documented as having a NAND-related
functionality in fact did not have such functionality. This commit
updates the pinctrl driver accordingly.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Cc: <stable@vger.kernel.org> # v3.15+
Fixes: ce3ed59d ("pinctrl: mvebu: add pin-muxing driver for the Marvell Armada 375")
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>