iommu: omap: add Kconfig OMAP dependency

Make CONFIG_OMAP_IOMMU depend on CONFIG_ARCH_OMAP so other
allmodconfig builds won't fail.

Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Ohad Ben-Cohen <ohad@wizery.com>
Signed-off-by: Joerg Roedel <joerg.roedel@amd.com>