interconnect: qcom: rpm: Use _optional func for provider clocks

It turned out that - very unfortunately - msm8996 needs a binding
update, adding 2 more clocks to the A2NoC node. Use the _optional
variant of devm_clk_get to make sure old DTs will still probe with
newer versions of the driver.

Signed-off-by: Konrad Dybcio <konrad.dybcio@linaro.org>
Reviewed-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>