clk: qcom: clk-rcg: Remove the src_freq and F_SLEW macro support

The current implementation of slewing RCG sources to custom src_freq
is flawed. It has issues with not setting the best_parent_rate and
incorrect voltage corner voting on behalf of the RCGs. Instead of
padding additional flags to work around this, remove the current
support.
If the RCG sources are to be slewed to frequencies that do not match
their divider configurations (CRC for instance), add a dummy fixed
divider to make things work. This is functionally more apt than
sending custom frequencies to sources.

Change-Id: Ife5d293439f04d72ddb0d7ac453a06d99c70f537
Signed-off-by: Deepak Katragadda <dkatraga@codeaurora.org>