pinctrl: ocelot: Add dependency on HAS_IOMEM

As usual the build fails on UM Linux because that thing does
not have IOMEM. Depend on HAS_IOMEM solves the build problem.

Cc: Alexandre Belloni <alexandre.belloni@bootlin.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>