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/Kconfig b/drivers/Kconfig
index c481e93..70993fd 100644
--- a/drivers/Kconfig
+++ b/drivers/Kconfig
@@ -46,6 +46,8 @@
 
 source "drivers/power/Kconfig"
 
+source "drivers/pwm/Kconfig"
+
 source "drivers/ram/Kconfig"
 
 source "drivers/remoteproc/Kconfig"