ARM: dt: use default early_init_dt_alloc_memory_arch

Now that ARM is using memblock instead of bootmem, the default version
of early_init_dt_alloc_memory_arch can be used.

Signed-off-by: Rob Herring <robh@kernel.org>
Cc: Russell King <linux@arm.linux.org.uk>
Cc: linux-arm-kernel@lists.infradead.org
Tested-by: Grant Likely <grant.likely@linaro.org>