clk: add stub clock driver

Add a stub clock driver which can be used to bind clock controllers
which aren't required for the platform to boot, but which are needed for
U-Boot drivers to work.

In addition, add a NOP parent driver to allow for binding the parent
nodes of the clock.

Initially this driver supports a Qualcomm platform where the MMC driver
tries to fetch the RPM clock controller, which is not actually required
for the device to work.

Reviewed-by: Neil Armstrong <neil.armstrong@linaro.org>
Acked-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>
Signed-off-by: Caleb Connolly <caleb.connolly@linaro.org>
diff --git a/drivers/clk/Kconfig b/drivers/clk/Kconfig
index d9d518d..18bd640 100644
--- a/drivers/clk/Kconfig
+++ b/drivers/clk/Kconfig
@@ -96,6 +96,13 @@
 	  Enable this option to add GPIO-controlled clock gate driver
 	  in U-Boot SPL.
 
+config CLK_STUB
+	bool "Stub clock driver"
+	depends on CLK
+	help
+	  Enable this to provide a stub clock driver for non-essential clock
+	  controllers.
+
 config CLK_BCM6345
 	bool "Clock controller driver for BCM6345"
 	depends on CLK && ARCH_BMIPS