clk: fixed-factor: Convert into a module platform driver

Adds support for fixed-factor clock providers which have not been
enabled via of_clk_init().

This is required by Device trees overlays that introduce clocks
providers.

Signed-off-by: Ricardo Ribalda Delgado <ricardo.ribalda@gmail.com>
[sboyd@codeaurora.org: Make new private function static, don't
check clk for NULL when unregistering in driver remove]
Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>