pwm: Add to device-drivers documentation

Add a short introductory text along with API documentation generated
from kerneldoc comments for the PWM framework.
Signed-off-by: NThierry Reding <thierry.reding@gmail.com>

Documentation/DocBook/device-drivers.tmpl

@@ -455,4 +455,31 @@ X!Ilib/fonts/fonts.c
 !Edrivers/hsi/hsi.c
   </chapter>
 
+  <chapter id="pwm">
+    <title>Pulse-Width Modulation (PWM)</title>
+    <para>
+      Pulse-width modulation is a modulation technique primarily used to
+      control power supplied to electrical devices.
+    </para>
+    <para>
+      The PWM framework provides an abstraction for providers and consumers
+      of PWM signals. A controller that provides one or more PWM signals is
+      registered as <structname>struct pwm_chip</structname>. Providers are
+      expected to embed this structure in a driver-specific structure. This
+      structure contains fields that describe a particular chip.
+    </para>
+    <para>
+      A chip exposes one or more PWM signal sources, each of which exposed
+      as a <structname>struct pwm_device</structname>. Operations can be
+      performed on PWM devices to control the period, duty cycle, polarity
+      and active state of the signal.
+    </para>
+    <para>
+      Note that PWM devices are exclusive resources: they can always only be
+      used by one consumer at a time.
+    </para>
+!Iinclude/linux/pwm.h
+!Edrivers/pwm/core.c
+  </chapter>
+
 </book>