clk: qcom: alpha: Add support for programming the PLL_FSM_LEGACY_MODE bit

This is used on at least SM6375 and its variations.

Signed-off-by: Konrad Dybcio <konrad.dybcio@somainline.org>
Signed-off-by: Bjorn Andersson <andersson@kernel.org>
Link: https://lore.kernel.org/r/20220921001303.56151-1-konrad.dybcio@somainline.org