OMAP4: iommu: fix clock name

0e433271 "OMAP4: clock: Fix clock names and align with hwmod names"
renamed ducati_ick to ipu_fck.

Update OMAP4's iommu_device accordingly, so that omap_iommu_probe
doesn't break when calling clk_get.

Signed-off-by: Ohad Ben-Cohen <ohad@wizery.com>