[POWERPC] Define of_read_ulong helper

There are various places where we want to extract an unsigned long
value from a device-tree property that can be 1 or 2 cells in length.
This replaces some open-coded calculations, and one place where we
assumed without checking that properties were the length we wanted,
with a little of_read_ulong() helper.

Signed-off-by: Paul Mackerras <paulus@samba.org>

arch/powerpc/kernel/time.c

@@ -870,9 +870,7 @@ static int __init get_freq(char *name, int cells, unsigned long *val)
 		fp = get_property(cpu, name, NULL);
 		if (fp) {
 			found = 1;
-			*val = 0;
-			while (cells--)
-				*val = (*val << 32) | *fp++;
+			*val = of_read_ulong(fp, cells);
 		}
 
 		of_node_put(cpu);