[POWERPC] Fix arch/ppc compilation - add typedef for pgtable_t

Commit 2f569afd ("CONFIG_HIGHPTE vs.
sub-page page tables.") breaks compilation of arch/ppc since it
introduces the pgtable_t type which was not added to arch/ppc.

This adds the missing typedef.
Signed-off-by: Stefan Roese <sr@denx.de>
Signed-off-by: Paul Mackerras <paulus@samba.org>

include/asm-ppc/page.h

@@ -125,6 +125,8 @@ extern __inline__ int get_order(unsigned long size)
 	return 32 - lz;
 }
 
+typedef struct page *pgtable_t;
+
 #endif /* __ASSEMBLY__ */
 
 #define VM_DATA_DEFAULT_FLAGS	(VM_READ | VM_WRITE | VM_EXEC | \