dm: led: Add support for blinking LEDs

Allow LEDs to be blinked if the driver supports it. Enable this for
sandbox so that the tests run.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Ziping Chen <techping.chan@gmail.com>
diff --git a/configs/sandbox_defconfig b/configs/sandbox_defconfig
index 7f3f5ac..9814ea3 100644
--- a/configs/sandbox_defconfig
+++ b/configs/sandbox_defconfig
@@ -83,6 +83,7 @@
 CONFIG_CROS_EC_KEYB=y
 CONFIG_I8042_KEYB=y
 CONFIG_LED=y
+CONFIG_LED_BLINK=y
 CONFIG_LED_GPIO=y
 CONFIG_DM_MAILBOX=y
 CONFIG_SANDBOX_MBOX=y