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/drivers/pwm/Makefile b/drivers/pwm/Makefile
index b037130..29d5991 100644
--- a/drivers/pwm/Makefile
+++ b/drivers/pwm/Makefile
@@ -15,4 +15,5 @@
 obj-$(CONFIG_PWM_EXYNOS)	+= exynos_pwm.o
 obj-$(CONFIG_PWM_IMX)		+= pwm-imx.o pwm-imx-util.o
 obj-$(CONFIG_PWM_ROCKCHIP)	+= rk_pwm.o
+obj-$(CONFIG_PWM_SANDBOX)	+= sandbox_pwm.o
 obj-$(CONFIG_PWM_TEGRA)		+= tegra_pwm.o