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>

common/fdt_support.c

@@ -579,7 +579,7 @@ void fdt_fixup_ethernet(void *fdt)
 
 			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;
 			}