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/tpm-common.c b/cmd/tpm-common.c
index a48c060..1d5442c 100644
--- a/cmd/tpm-common.c
+++ b/cmd/tpm-common.c
@@ -302,7 +302,7 @@
 	int rc;
 
 	if (argc == 2) {
-		num = simple_strtoul(argv[1], NULL, 10);
+		num = dectoul(argv[1], NULL);
 
 		rc = tpm_set_device(num);
 		if (rc)