clk: qcom: clk-rcg2: Introduce a cfg offset for RCGs

The RCG CFG/M/N/D register base could be at a different offset than the CMD
register, so introduce a cfg_offset to identify the offset with respect to
the CMD register.

Signed-off-by: Taniya Das <tdas@codeaurora.org>
Signed-off-by: Anu Ramanathan <anur@codeaurora.org>
Signed-off-by: Shawn Guo <shawn.guo@linaro.org>