ARM: dts: BCM5301X: Fix UARTs on bcm953012k

The UARTs are outputting garbage on the console.  This is due to a speed
issue.  We can simply use the clock speed (which is now defined in the
DTSI file) and everything works fine.

Signed-off-by: Jon Mason <jon.mason@broadcom.com>
Fixes: cdc36b22 ("ARM: dts: enable clock support for BCM5301X")
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>