xen/arm: dom0_build: Remove dead code in allocate_memory

xen/arm: dom0_build: Remove dead code in allocate_memory

The code to allocate memory when dom0 does not use direct mapping is
relying on the presence of memory node in the DT.

However, they are not present when booting using UEFI or when using
ACPI.

Rather than fixing the code, remove it because dom0 is always direct
memory mapped and therefore the code is never tested. Also add a
check to avoid disabling direct memory mapped and not implementing
the associated RAM bank allocation.

Signed-off-by: Julien Grall <julien.grall@arm.com>
Acked-by: Stefano Stabellini <sstabellini@kernel.org>