MIPS: init: Prevent adding memory before PHYS_OFFSET

On some SGI machines (IP28 and IP30) a small region of memory is mirrored
to pyhsical address 0 for exception vectors while rest of the memory
is reachable at a higher physical address. ARC PROM marks this
region as reserved, but with commit a94e4f24 ("MIPS: init: Drop
boot_mem_map") this chunk is used, when searching for start of ram,
which breaks at least IP28 and IP30 machines. To fix this
add_region_memory() checks for start address < PHYS_OFFSET and ignores
these chunks.

Fixes: a94e4f24 ("MIPS: init: Drop boot_mem_map")
Signed-off-by: Thomas Bogendoerfer <tbogendoerfer@suse.de>
Signed-off-by: Paul Burton <paul.burton@mips.com>
Cc: Ralf Baechle <ralf@linux-mips.org>
Cc: James Hogan <jhogan@kernel.org>
Cc: linux-mips@vger.kernel.org
Cc: linux-kernel@vger.kernel.org

arch/mips/kernel/setup.c

@@ -108,6 +108,9 @@ void __init add_memory_region(phys_addr_t start, phys_addr_t size, long type)
 		return;
 	}
 
+	if (start < PHYS_OFFSET)
+		return;
+
 	memblock_add(start, size);
 	/* Reserve any memory except the ordinary RAM ranges. */
 	switch (type) {