memblock: Add array resizing support

When one of the array gets full, we resize it. After much thinking and
a few iterations of that code, I went back to on-demand resizing using
the (new) internal memblock_find_base() function, which is pretty much what
Yinghai initially proposed, though there some differences in the details.

To work this relies on the default alloc limit being set sensibly by
the architecture.

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