clk: qcom: smd-rpm: Keep one rpm handle for all clocks

For no apparent reason (as there's just one RPM per SoC), all clocks
currently store a copy of a pointer to smd_rpm. Introduce a single,
global one to save up on space in each clk definition.

bloat-o-meter reports:

Total: Before=41887, After=40843, chg -2.49%

Signed-off-by: Konrad Dybcio <konrad.dybcio@linaro.org>
Signed-off-by: Bjorn Andersson <andersson@kernel.org>
Link: https://lore.kernel.org/r/20230501130400.107771-1-konrad.dybcio@linaro.org