remoteproc: do not require an iommu

Not all remote processors employ an IOMMU, so do not error out
on !iommu_present().

Note: we currently still use iommu_present() to tell whether we need
to configure an IOMMU or not. That works for simple cases, but will
easily fail with more complicated ones (e.g. where an IOMMU exists,
but not all remote processors use it). When those use cases show up,
we will solve them by introducing something like remoteproc hw
capabilities.

[ohad@wizery.com: write commit log]

Signed-off-by: Mark Grosen <mgrosen@ti.com>
Signed-off-by: Ohad Ben-Cohen <ohad@wizery.com>