clk: add stub clock driver

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>