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/optee_rpmb.c b/cmd/optee_rpmb.c
index 0d6b1cb..e0e44bb 100644
--- a/cmd/optee_rpmb.c
+++ b/cmd/optee_rpmb.c
@@ -195,7 +195,7 @@
 		return CMD_RET_USAGE;
 
 	name = argv[1];
-	bytes = simple_strtoul(argv[2], &endp, 10);
+	bytes = dectoul(argv[2], &endp);
 	if (*endp && *endp != '\n')
 		return CMD_RET_USAGE;