[POWERPC] Add "is_power_of_2" checking to log2.h.

Add the inline function "is_power_of_2()" to log2.h, where the value
zero is *not* considered to be a power of two.

Signed-off-by: Robert P. J. Day <rpjday@mindspring.com>
Acked-by: Kumar Gala <galak@kernel.crashing.org>
Signed-off-by: Paul Mackerras <paulus@samba.org>

arch/powerpc/mm/pgtable_32.c

@@ -294,11 +294,8 @@ void __init mapin_ram(void)
 	}
 }
 
-/* is x a power of 2? */
-#define is_power_of_2(x)	((x) != 0 && (((x) & ((x) - 1)) == 0))
-
 /* is x a power of 4? */
-#define is_power_of_4(x)	((x) != 0 && (((x) & (x-1)) == 0) && (ffs(x) & 1))
+#define is_power_of_4(x)	is_power_of_2(x) && (ffs(x) & 1))
 
 /*
  * Set up a mapping for a block of I/O.