ARM: dts: armada-38x: adjust board name and compatible for Armada 388 GP

As the name of the Device Tree file name suggests, the Armada 388 GP
really contains an Armada 388 SoC, so this commit updates the board
name and compatible string in the Device Tree file.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Gregory CLEMENT <gregory.clement@free-electrons.com>