remoteproc: da8xx: don't select CMA on no-MMU

We can only use CMA on systems that have an MMU, because of
the requirement to use memory migration. NOMMU systems are
rather constrained to start with, but it seems reasonable
to assume that DMA allocations can still succeed in the
constrained case for remoteproc on NOMMU, so this patch
changes the da8xx implementation to not rely on CMA when
the MMU is disabled.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Cc: Ohad Ben-Cohen <ohad@wizery.com>
Cc: Robert Tivy <rtivy@ti.com>