OMAP: clock: Add dummy round_rate and set_rate functions

Add functions which are going to be used by dummy virtual clocks:
long clk_dummy_round_rate(struct clk *clk, unsigned long rate)
int clk_dummy_set_rate(struct clk *clk, unsigned long rate)

Change-Id: I9bb1e886888a117548d62ef2edd9598bce2723e6
Signed-off-by: Taras Kondratiuk <taras@ti.com>