clock-snapdragon: Add clk_rcg_set_rate() with mnd_width=0

Add clk_rcg_set_rate() which allows to configure clocks without programming
MND values. This is required for configuring I2C clocks on QCS404.

Co-developed-by: Mike Worsfold <mworsfold@impinj.com>
Signed-off-by: Mike Worsfold <mworsfold@impinj.com>
Signed-off-by: Sumit Garg <sumit.garg@linaro.org>