kerneldoc for <linux/clk.h>

Add <linux/clk.h> to the generated kerneldoc, with some overview
to go along with those per-function descriptions.

Signed-off-by: David Brownell <[e-mail]>
Cc: Russell King <[e-mail]>
Cc: Alessandro Zummo <[e-mail]>
Cc: "Randy.Dunlap" <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

Documentation/DocBook/kernel-api.tmpl

···
 !Edrivers/i2c/i2c-core.c
   </chapter>
 
+  <chapter id="clk">
+     <title>Clock Framework</title>
+
+     <para>
+	The clock framework defines programming interfaces to support
+	software management of the system clock tree.
+	This framework is widely used with System-On-Chip (SOC) platforms
+	to support power management and various devices which may need
+	custom clock rates.
+	Note that these "clocks" don't relate to timekeeping or real
+	time clocks (RTCs), each of which have separate frameworks.
+	These <structname>struct clk</structname> instances may be used
+	to manage for example a 96 MHz signal that is used to shift bits
+	into and out of peripherals or busses, or otherwise trigger
+	synchronous state machine transitions in system hardware.
+     </para>
+
+     <para>
+	Power management is supported by explicit software clock gating:
+	unused clocks are disabled, so the system doesn't waste power
+	changing the state of transistors that aren't in active use.
+	On some systems this may be backed by hardware clock gating,
+	where clocks are gated without being disabled in software.
+	Sections of chips that are powered but not clocked may be able
+	to retain their last state.
+	This low power state is often called a <emphasis>retention
+	mode</emphasis>.
+	This mode still incurs leakage currents, especially with finer
+	circuit geometries, but for CMOS circuits power is mostly used
+	by clocked state changes.
+     </para>
+
+     <para>
+	Power-aware drivers only enable their clocks when the device
+	they manage is in active use.  Also, system sleep states often
+	differ according to which clock domains are active:  while a
+	"standby" state may allow wakeup from several active domains, a
+	"mem" (suspend-to-RAM) state may require a more wholesale shutdown
+	of clocks derived from higher speed PLLs and oscillators, limiting
+	the number of possible wakeup event sources.  A driver's suspend
+	method may need to be aware of system-specific clock constraints
+	on the target sleep state.
+     </para>
+
+     <para>
+        Some platforms support programmable clock generators.  These
+	can be used by external chips of various kinds, such as other
+	CPUs, multimedia codecs, and devices with strict requirements
+	for interface clocking.
+     </para>
+
+!Iinclude/linux/clk.h
+  </chapter>
+
 </book>