of/flattree: Don't assume HAVE_LMB

We don't always have lmb available, so make arches provide an
early_init_dt_alloc_memory_arch() to handle the allocation of
memory in the fdt code.

When we don't have lmb.h included, we need asm/page.h for __va.

Signed-off-by: Jeremy Kerr <jeremy.kerr@canonical.com>
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>
Acked-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Acked-by: Michal Simek <monstr@monstr.eu>