memblock: Remove memblock_type.size and add memblock.memory_size instead

Right now, both the "memory" and "reserved" memblock_type structures have
a "size" member. It represents the calculated memory size in the former
case and is unused in the latter.

This moves it out to the main memblock structure instead

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>