dm: test: Add tests for the clk uclass

Add tests of each API call using a sandbox clock device.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/configs/sandbox_defconfig b/configs/sandbox_defconfig
index 7aea381..29e7b5b 100644
--- a/configs/sandbox_defconfig
+++ b/configs/sandbox_defconfig
@@ -45,3 +45,4 @@
 CONFIG_UT_TIME=y
 CONFIG_UT_DM=y
 CONFIG_UT_ENV=y
+CONFIG_CLK=y