dt/fdt: add empty versions of early_init_dt_*_memory_arch

With the addition of commit 0166dc11 (of: make CONFIG_OF user
selectable), architectures which don't enable memblock and don't
have their own early_init_dt_*_memory_arch implementations will break
when CONFIG_OF is enabled.

Really, we should have better separation of CONFIG_OF and
CONFIG_OF_EARLY_FLATTREE, but doing that will require quite a bit of
shuffling of architecture code. That will have to wait for another day.

Signed-off-by: Rob Herring <robh@kernel.org>
Cc: Grant Likely <grant.likely@linaro.org>