clk: update clk API documentation to clarify clk_round_rate()

The idea is that rate = clk_round_rate(clk, r) is equivalent to:

	clk_set_rate(clk, r);
	rate = clk_get_rate(clk);

except that clk_round_rate() does not change the hardware in any way.

Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>