ath79: enable UART in SoC DTSI files

The uart node is enabled on all devices except one (GL-USB150 *).
Thus, let's not have a few hundred nodes to enable it, but do not
disable it in the first place.

Where the majority of devices is using it, also move the serial0
alias to the DTSI.

*) Since GL-USB150 even defines serial0 alias, the missing uart
   is probably just a mistake. Anyway, disable it for now so this
   patch stays cosmetic.

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