mips: dts: realtek: Clean up CPU clocks

The referenced CPU clock does not require any additional #clock-cells,
so drop the extraneous '0' in the referenced CPU clock.

The binding for MIPS cpus also does not allow for the clock-names
property, so just drop it.

This resolves some error message from 'dtbs_check':
    cpu@0: clocks: [[4], [0]] is too long
    'clock-names' does not match any of the regexes: 'pinctrl-[0-9]+'

Signed-off-by: Sander Vanheule <sander@svanheule.net>
Reviewed-by: Chris Packham <chris.packham@alliedtelesis.co.nz>
Tested-by: Chris Packham <chris.packham@alliedtelesis.co.nz> # For RTL9302C
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>