mn10300: Declare __pfn_to_phys() to fix build error

__pfn_to_phys() was introduced with commit 8b79eb6f5b53e ("libnvdimm, pfn,
pmem: allocate memmap array in persistent memory") but not declared for mn10300. This results in the following build error.

mm/page_alloc.c: In function 'memmap_init_zone':
mm/page_alloc.c:4610:9: error: implicit declaration of function '__pfn_to_phys'

---
 arch/mn10300/include/asm/page.h | 1 +
 1 file changed, 1 insertion(+)

index 8288e124165b..3810a6f740fd 100644
@@ -107,6 +107,7 @@ static inline int get_order(unsigned long size)
 #define pfn_to_kaddr(pfn)	__va((pfn) << PAGE_SHIFT)
 #define pfn_to_page(pfn)	(mem_map + ((pfn) - __pfn_disp))
 #define page_to_pfn(page)	((unsigned long)((page) - mem_map) + __pfn_disp)
+#define __pfn_to_phys(pfn)	PFN_PHYS(pfn)
 
 #define pfn_valid(pfn)					\
 ({							\

2.1.4