dm: gpio: Add gpio_requestf() helper for printf() strings

Add a helper which permits a printf()-style format string for the requester
string.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/doc/driver-model/README.txt b/doc/driver-model/README.txt
index 1ff454a..f4395c1 100644
--- a/doc/driver-model/README.txt
+++ b/doc/driver-model/README.txt
@@ -95,7 +95,7 @@
 You should see something like this:
 
     <...U-Boot banner...>
-    Running 26 driver model tests
+    Running 27 driver model tests
     Test: dm_test_autobind
     Test: dm_test_autoprobe
     Test: dm_test_bus_children
@@ -117,6 +117,7 @@
     Test: dm_test_gpio
     extra-gpios: get_value: error: gpio b5 not reserved
     Test: dm_test_gpio_anon
+    Test: dm_test_gpio_requestf
     Test: dm_test_leak
     Test: dm_test_lifecycle
     Test: dm_test_operations