ARM: dts: mvebu: armada-390: add missing compatibility string and bracket

The armada-390.dtsi was broken since the first patch which adds Device Tree
files for Armada 39x SoC was introduced.

Signed-off-by: Grzegorz Jaszczyk <jaz@semihalf.com>
Acked-by: Gregory CLEMENT <gregory.clement@free-electrons.com>
Cc: <stable@vger.kernel.org> # 4.0+
Fixes 538da83d

 ("ARM: mvebu: add Device Tree files for Armada 39x SoC and board")

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