lib: memory_alloc: Support 64-bit physical addresses

The current memory allocation code does not support physical
addresses above 32-bits, meaning that LPAE is not supported.
Change the types of the code internally to support 64-bit
physical addresses. Full 64-bit virtual addresses may not be
fully supported so this code will still need an update when
that happens.

Change-Id: I0c4a7b76784981ffcd7f4776d51e05f3592bb7b8
Signed-off-by: Laura Abbott <lauraa@codeaurora.org>