ARM: dts: armada-38x: Add generic compatible to UART nodes

Add generic compatible string "ns16550a" to serial port nodes of Armada
38x.

This makes it possible to use earlycon.

Fixes: 0d3d96ab ("ARM: mvebu: add Device Tree description of the Armada 380/385 SoCs")
Signed-off-by: Pali Rohár <pali@kernel.org>
Signed-off-by: Marek Behún <kabel@kernel.org>
Signed-off-by: Gregory CLEMENT <gregory.clement@bootlin.com>