clk: Provide option for clk_get_rate to issue hw for new rate

By using CLK_GET_RATE_NOCACHE flag, we tell the clk_get_rate API to
issue the hw for an updated clock rate. This can be used for a clock
which rate may be updated without a client necessary modifying it.

Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>
Signed-off-by: Mike Turquette <mturquette@linaro.org>