clk: qcom: branch: Add helper functions for setting retain bits

Most Qualcomm branch clocks come with a pretty usual set of bits that
can enable memory retention by means of not turning off parts of the
memory logic. Add them to the common header file and introduce helper
functions for setting them instead of using magic writes.

Signed-off-by: Konrad Dybcio <konrad.dybcio@linaro.org>
Signed-off-by: Bjorn Andersson <andersson@kernel.org>
Link: https://lore.kernel.org/r/20230208091340.124641-2-konrad.dybcio@linaro.org