clk: qcom: clk-dummy: Add a dummy clock provider

Add a dummy clock provider that registers a simple callback that
in turn always returns the dummy clock for any clk_get call. This is useful
for unimplemented clocks.

Change-Id: I08fcb174fd0e0c49f8069e106b48597bcdfe847d
Signed-off-by: Taniya Das <tdas@codeaurora.org>