clk: fixed_rate: add API for directly registering fixed rate clocks

Current driver only supports registering fixed rate clocks from DT. Add
new API which makes it possible to register fixed rate clocks directly
from e.g. platform specific clock drivers.

Reviewed-by: Peng Fan <peng.fan@nxp.com>
Signed-off-by: Tero Kristo <t-kristo@ti.com>
Signed-off-by: Tero Kristo <kristo@kernel.org>