clk: add clk_valid()

Add clk_valid() to check for optional clocks are valid.
Call clk_valid() in test/dm/clk.c and add relevant test routine to
sandbox clk tests.

Signed-off-by: Fabrice Gasnier <fabrice.gasnier@st.com>
Reviewed-by: Simon Glass <sjg@chromium.org>

test/dm/clk.c

@@ -28,6 +28,7 @@ static int dm_test_clk(struct unit_test_state *uts)
 	ut_assertok(uclass_get_device_by_name(UCLASS_MISC, "clk-test",
 					      &dev_test));
 	ut_assertok(sandbox_clk_test_get(dev_test));
+	ut_assertok(sandbox_clk_test_valid(dev_test));
 
 	ut_asserteq(1234,
 		    sandbox_clk_test_get_rate(dev_test,