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/mem.c b/cmd/mem.c
index 0978df5..b751138 100644
--- a/cmd/mem.c
+++ b/cmd/mem.c
@@ -189,7 +189,7 @@
 	if (argc < 4)
 		return CMD_RET_USAGE;
 
-	count = simple_strtoul(argv[3], NULL, 10);
+	count = dectoul(argv[3], NULL);
 
 	for (;;) {
 		do_mem_md (NULL, 0, 3, argv);
@@ -217,7 +217,7 @@
 	if (argc < 4)
 		return CMD_RET_USAGE;
 
-	count = simple_strtoul(argv[3], NULL, 10);
+	count = dectoul(argv[3], NULL);
 
 	for (;;) {
 		do_mem_mw (NULL, 0, 3, argv);