[media] V4L2: add v4l2-clock helpers to register and unregister a fixed-rate clock

Many bridges and video host controllers supply fixed rate always on clocks
to their I2C devices. This patch adds two simple helpers to register and
unregister such a clock.

Signed-off-by: Guennadi Liakhovetski <g.liakhovetski@gmx.de>
Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>

include/media/v4l2-clk.h

@@ -15,6 +15,7 @@
 #define MEDIA_V4L2_CLK_H
 
 #include <linux/atomic.h>
+#include <linux/export.h>
 #include <linux/list.h>
 #include <linux/mutex.h>
 
@@ -51,4 +52,17 @@ void v4l2_clk_disable(struct v4l2_clk *clk);
 unsigned long v4l2_clk_get_rate(struct v4l2_clk *clk);
 int v4l2_clk_set_rate(struct v4l2_clk *clk, unsigned long rate);
 
+struct module;
+
+struct v4l2_clk *__v4l2_clk_register_fixed(const char *dev_id,
+		const char *id, unsigned long rate, struct module *owner);
+void v4l2_clk_unregister_fixed(struct v4l2_clk *clk);
+
+static inline struct v4l2_clk *v4l2_clk_register_fixed(const char *dev_id,
+							const char *id,
+							unsigned long rate)
+{
+	return __v4l2_clk_register_fixed(dev_id, id, rate, THIS_MODULE);
+}
+
 #endif