memblock/arm: pfn_valid uses memblock_is_memory()

The implementation is pretty much similar. There is a -small- added
overhead by having another function call and the address shift.

If that becomes a concern, I suppose we could actually have memblock
itself expose a memblock_pfn_valid() which then ARM can use directly
with an appropriate #define...

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