clk: qcom: Change default behaviour for clk_round_rate

If a clock supports multiple rates, the round_rate op should be
defined. For clocks which only support one rate, this op is no
longer required.

Change-Id: I68617537a0c119885941a5c6f478d31f530a3c0b
Signed-off-by: Patrick Daly <pdaly@codeaurora.org>