memblock: Add arch function to control coalescing of memblock memory regions

Some archs such as ARM want to avoid coalescing accross things such
as the lowmem/highmem boundary or similar. This provides the option
to control it via an arch callback for which a weak default is provided
which always allows coalescing.

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

include/linux/memblock.h

@@ -72,6 +72,8 @@ extern void memblock_dump_all(void);
 
 /* Provided by the architecture */
 extern phys_addr_t memblock_nid_range(phys_addr_t start, phys_addr_t end, int *nid);
+extern int memblock_memory_can_coalesce(phys_addr_t addr1, phys_addr_t size1,
+				   phys_addr_t addr2, phys_addr_t size2);
 
 /**
  * memblock_set_current_limit - Set the current allocation limit to allow