arm/arm64: setup: Consolidate memory layout assumptions

Keep as much memory layout assumptions as possible in init::start
and a single setup function. This prepares us for calling setup()
from different start functions which have been linked with different
linker scripts. To do this, stacktop is only referenced from
init::start, making freemem_start a parameter to setup(). We also
split mem_init() into three parts, one that populates the mem regions
per the DT, one that populates the mem regions per assumptions,
and one that does the mem init. The concept of a primary region
is dropped, but we add a sanity check for the absence of memory
holes, because we don't know how to deal with them yet.

Reviewed-by: Nikos Nikoleris <nikos.nikoleris@arm.com>
Reviewed-by: Alexandru Elisei <alexandru.elisei@arm.com>
Tested-by: Alexandru Elisei <alexandru.elisei@arm.com>
Signed-off-by: Andrew Jones <drjones@redhat.com>