ipq806x: move serial0 to DTSI files

ipq806x: move serial0 to DTSI files

All device DTS files in the target set the serial0 property to the
same value (*). So, let's move the definitions to the DTSI files.

That's also where the kernel defines it (qcom-ipq8064-v1.0.dtsi).

* The only exception is ipq8064-db149, which defines
  "serial0 = &uart2;", but inside a block called "alias" instead of
  "aliases". It must be assumed that this is broken anyway, so we
  don't touch it here.

Signed-off-by: Adrian Schmutzler <freifunk@adrianschmutzler.de>