arm64: Move unflatten_device_tree() call earlier.

In order to extract NUMA information from the device tree, we need to
have the tree in its unflattened form.

Move the call to bootmem_init() in the tail of paging_init() into
setup_arch, and adjust header files so that its declaration is
visible.

Move the unflatten_device_tree() call between the calls to
paging_init() and bootmem_init().  Follow on patches add NUMA handling
to bootmem_init().

Signed-off-by: David Daney <david.daney@cavium.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>