msm: clock: Support dummy clocks

Implement a dummy clock driver and macro in the spirit of
CLK_PCOM(). This is useful on targets where a pclk doesn't exist
and we want to keep drivers sane by providing such clocks.

Reviewed-by: Saravana Kannan <skannan@codeaurora.org>
Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>
Signed-off-by: David Brown <davidb@codeaurora.org>