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/common/fdt_support.c b/common/fdt_support.c
index 240f1e5..4341d84 100644
--- a/common/fdt_support.c
+++ b/common/fdt_support.c
@@ -579,7 +579,7 @@
 
 			for (j = 0; j < 6; j++) {
 				mac_addr[j] = tmp ?
-					      simple_strtoul(tmp, &end, 16) : 0;
+					      hextoul(tmp, &end) : 0;
 				if (tmp)
 					tmp = (*end) ? end + 1 : end;
 			}