mips: dts: realtek: Replace uart clock property

Add a fixed clock to define the clock frequency of the Lexra bus and use
this for the two uart nodes instead of a separate clock-frequency
property.

Signed-off-by: Sander Vanheule <sander@svanheule.net>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>