omap-serial: Use default clock speed (48Mhz) if not specified

Use a default clock speed of 48Mhz, instead of ending up with 0,
if platforms fail to specify a valid clock speed.

Signed-off-by: Rajendra Nayak <rnayak@ti.com>
Reviewed-by: Rob Herring <rob.herring@calxeda.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>