arm/arm64: mmu: Remove memory layout assumptions

Rather than making too many assumptions about the memory layout
in mmu code, just set up the page tables per the memory regions
(which means putting all the memory layout assumptions in setup).
To ensure we get the right default flags set we need to split the
primary region into two regions for code and data.

We still only expect the primary regions to be present, but the
next patch will remove that assumption too.

(Unfortunately we still have an assumption in setup_mmu. We assume
 the range 3G-4G is available for the virtual memory allocator. We'll
 need to remove that assumption as well with another patch in order
 to support arbitrary memory maps.)

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>