clk/qcom: add mnd_width to clk_rcg_set_rate_mnd()

This property is needed on some platforms to ensure that only the
relevant bits are set in the M/N/D registers.

Reviewed-by: Sumit Garg <sumit.garg@linaro.org>
Signed-off-by: Caleb Connolly <caleb.connolly@linaro.org>