global: Convert simple_strtoul() with decimal to dectoul()

It is a pain to have to specify the value 10 in each call. Add a new
dectoul() function and update the code to use it.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/cmd/cros_ec.c b/cmd/cros_ec.c
index abda5d6..a40f589 100644
--- a/cmd/cros_ec.c
+++ b/cmd/cros_ec.c
@@ -501,11 +501,11 @@
 
 		if (argc < 3)
 			return CMD_RET_USAGE;
-		index = simple_strtoul(argv[2], &endp, 10);
+		index = dectoul(argv[2], &endp);
 		if (*argv[2] == 0 || *endp != 0)
 			return CMD_RET_USAGE;
 		if (argc > 3) {
-			state = simple_strtoul(argv[3], &endp, 10);
+			state = dectoul(argv[3], &endp);
 			if (*argv[3] == 0 || *endp != 0)
 				return CMD_RET_USAGE;
 			ret = cros_ec_set_ldo(dev, index, state);