mips: dts: realtek: use "serial" instead of "uart" in node name

Update the node name for the UARTs to resolve the following dtbs_check
complaints:

  uart@2000: $nodename:0: 'uart@2000' does not match '^serial(@.*)?$'
  uart@2100: $nodename:0: 'uart@2100' does not match '^serial(@.*)?$'

Signed-off-by: Chris Packham <chris.packham@alliedtelesis.co.nz>
Reviewed-by: Marek Behún <kabel@kernel.org>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>