Blackfin: NOMPU: skip DMA ICPLB hole when it is redundant

Normally there is no user-reserved memory after the DMA region which means
there is no user-reserved ICPLB coverage.  So the DMA hole can be covered
by the large hole that is always added to cover up to the async bank.  We
only need an explicit DMA whole when we also add an explicit mapping for
the user-reserved memory.

Signed-off-by: Mike Frysinger <vapier@gentoo.org>