serial: 8250_omap: Use clk_get_rate() to obtain fclk frequency

8250_omap driver uses clock-frequency DT property to obtain functional
clk frequency. This is not ideal as users need to calculate functional
clk frequency offline and populate it in DT.
Therefore add support to obtain functional clock frequency using clk
APIs when clock-frequency DT property is not defined.

Suggested-by: Lokesh Vutla <lokeshvutla@ti.com>
Signed-off-by: Vignesh R <vigneshr@ti.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>