ARM: dts: tango4: Make /serial compatible with ns16550a

ARM: dts: tango4: Make /serial compatible with ns16550a

ralink,rt2880-uart is compatible with ns16550a and all other
instances of RT2880 UART nodes include it in the compatible property.
Add it also here, to make the binding schema simpler.

Signed-off-by: Lubomir Rintel <lkundrak@v3.sk>
Reviewed-by: Rob Herring <robh@kernel.org>
Acked-by: Mans Rullgard <mans@mansr.com>
Link: https://lore.kernel.org/r/20200320174107.29406-8-lkundrak@v3.sk
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>