dm: pwm: Add a PWM uclass

Add a uclass that supports Pulse Width Modulation (PWM) devices. It
provides methods to enable/disable and configure the device.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/drivers/pwm/Makefile b/drivers/pwm/Makefile
index c0c4883..d1b15e5 100644
--- a/drivers/pwm/Makefile
+++ b/drivers/pwm/Makefile
@@ -10,4 +10,5 @@
 
 #ccflags-y += -DDEBUG
 
+obj-$(CONFIG_DM_PWM) += pwm-uclass.o
 obj-$(CONFIG_PWM_IMX) += pwm-imx.o pwm-imx-util.o