msm: clock-dummy: Rate-setable independent dummy clocks

Previously dummy clocks share same underlying clk struct.  clk_get_rate()
always returns 0.  Add a new macro DEFINE_CLK_DUMMY to create different
dummy clocks.  These clocks remember last set rate and return it for
clk_get_rate().  This is necessary for fake clocks to return desired
rate in virtio.

For any dummy clock that might need to handle clk_get_rate(), the new
macro is recommended.

Change-Id: I3a279edf6e41a0ae2777400adf992a9f26d77287
Signed-off-by: Junjie Wu <junjiew@codeaurora.org>