dm: sandbox: pwm: Add a basic pwm test

Unfortunately a test for the PWM uclass was not included when it was
submitted. This was noticed when trying to add more functionality:

   http://patchwork.ozlabs.org/patch/748172/

Add a simple test to get us started.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/configs/sandbox_defconfig b/configs/sandbox_defconfig
index 9814ea3..cdaf3bb 100644
--- a/configs/sandbox_defconfig
+++ b/configs/sandbox_defconfig
@@ -173,3 +173,5 @@
 CONFIG_UT_TIME=y
 CONFIG_UT_DM=y
 CONFIG_UT_ENV=y
+CONFIG_DM_PWM=y
+CONFIG_PWM_SANDBOX=y
diff --git a/configs/sandbox_noblk_defconfig b/configs/sandbox_noblk_defconfig
index bba7443..087c7a8 100644
--- a/configs/sandbox_noblk_defconfig
+++ b/configs/sandbox_noblk_defconfig
@@ -175,3 +175,5 @@
 CONFIG_UT_TIME=y
 CONFIG_UT_DM=y
 CONFIG_UT_ENV=y
+CONFIG_DM_PWM=y
+CONFIG_PWM_SANDBOX=y
diff --git a/configs/sandbox_spl_defconfig b/configs/sandbox_spl_defconfig
index 6fe2125..ffdd15b 100644
--- a/configs/sandbox_spl_defconfig
+++ b/configs/sandbox_spl_defconfig
@@ -179,3 +179,5 @@
 CONFIG_UT_TIME=y
 CONFIG_UT_DM=y
 CONFIG_UT_ENV=y
+CONFIG_DM_PWM=y
+CONFIG_PWM_SANDBOX=y