ARM: dts: mmp*: Fix serial port names

A preferred node name for serial ports is "serial":

  mmp2-olpc-xo-1-75.dt.yaml: uart@d4030000: $nodename:0: 'uart@d4030000'
      does not match '^serial(@[0-9a-f,]+)*$'
  ...

Signed-off-by: Lubomir Rintel <lkundrak@v3.sk>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20200320174107.29406-6-lkundrak@v3.sk


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>