clk: add a clk debug command

Add a command and associated op to allow clock drivers to implement
their own debug commands. Some clock controller hardware implements
debug features like support for measuring clock output rates. This
allows using a common entrypoint to driver-specific clock debugging
functionality.

Signed-off-by: Caleb Connolly <caleb.connolly@linaro.org>
diff --git a/include/clk.h b/include/clk.h
index bfdad96..3c19e82 100644
--- a/include/clk.h
+++ b/include/clk.h
@@ -551,6 +551,8 @@
 
 void clk_dump_clks(struct udevice *dev);
 
+void clk_debug_clks(struct udevice *dev, int argc, char *const argv[]);
+
 /**
  * clk_disable_bulk() - Disable (turn off) all clocks in a clock bulk struct.
  * @bulk:	A clock bulk struct that was previously successfully requested