of: create default early_init_dt_add_memory_arch

Create a weak version of early_init_dt_add_memory_arch which uses
memblock. This will unify all architectures except ones with custom
memory bank structs.

Signed-off-by: Rob Herring <rob.herring@calxeda.com>
Acked-by: Catalin Marinas <catalin.marinas@arm.com>
Cc: Will Deacon <will.deacon@arm.com>
Cc: Michal Simek <monstr@monstr.eu>
Cc: Jonas Bonn <jonas@southpole.se>
Acked-by: Grant Likely <grant.likely@linaro.org>
Cc: linux-arm-kernel@lists.infradead.org
Cc: microblaze-uclinux@itee.uq.edu.au
Cc: linux@lists.openrisc.net
Cc: devicetree@vger.kernel.org