global: Convert simple_strtoul() with hex to hextoul()

It is a pain to have to specify the value 16 in each call. Add a new
hextoul() function and update the code to use it.

Add a proper comment to simple_strtoul() while we are here.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/drivers/qe/qe.c b/drivers/qe/qe.c
index 97883ff..bd0a10c 100644
--- a/drivers/qe/qe.c
+++ b/drivers/qe/qe.c
@@ -794,7 +794,7 @@
 		return cmd_usage(cmdtp);
 
 	if (strcmp(argv[1], "fw") == 0) {
-		addr = simple_strtoul(argv[2], NULL, 16);
+		addr = hextoul(argv[2], NULL);
 
 		if (!addr) {
 			printf("Invalid address\n");
@@ -807,7 +807,7 @@
 		 */
 
 		if (argc > 3) {
-			ulong length = simple_strtoul(argv[3], NULL, 16);
+			ulong length = hextoul(argv[3], NULL);
 			struct qe_firmware *firmware = (void *)addr;
 
 			if (length != be32_to_cpu(firmware->header.length)) {