serial: lantiq: Add CCF support

Previous implementation uses platform-dependent API to get the clock.
Those functions are not available for other SoC which uses the same IP.
The CCF (Common Clock Framework) have an abstraction based APIs for
clock. In future, the platform specific code will be removed when the
legacy soc use CCF as well.
Change to use CCF APIs to get clock and rate. So that different SoCs
can use the same driver.

Signed-off-by: Songjun Wu <songjun.wu@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>