clk: qcom: clk-rcg2: document calc_rate function

Update calc_rate docs to reflect, that pre_div
is not pure divisor, but a register value, and requires conversion.

Signed-off-by: Dzmitry Sankouski <dsankouski@gmail.com>
Link: https://lore.kernel.org/r/20241118-starqltechn_integration_upstream-v8-1-ac8e36a3aa65@gmail.com


Signed-off-by: Bjorn Andersson <andersson@kernel.org>