dm: core: Add a function to look up a uclass by name

Each uclass has a driver name which we can use to look up the uclass. This
is useful for logging, where the uclass ID is used as the category.

Add a function to handle this, as well as a test.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/test/dm/core.c b/test/dm/core.c
index 50ee41b..052bf8f 100644
--- a/test/dm/core.c
+++ b/test/dm/core.c
@@ -862,3 +862,12 @@
 	return 0;
 }
 DM_TEST(dm_test_device_get_uclass_id, DM_TESTF_SCAN_PDATA);
+
+static int dm_test_uclass_names(struct unit_test_state *uts)
+{
+	ut_asserteq_str("test", uclass_get_name(UCLASS_TEST));
+	ut_asserteq(UCLASS_TEST, uclass_get_by_name("test"));
+
+	return 0;
+}
+DM_TEST(dm_test_uclass_names, DM_TESTF_SCAN_PDATA);