drivers: serial: serial_omap: populate default clock frequency when not found in dt

drivers: serial: serial_omap: populate default clock frequency when not found in dt

In some platforms like am437x, serial node is not populated with
clock-frequency node. So in that case have a default clock-clock
frequency.

Signed-off-by: Mugunthan V N <mugunthanvnm@ti.com>
Reviewed-by: Lokesh Vutla <lokeshvutla@ti.com>