ARM: dts: armada-39x: Fix compatible string for gpios

Armada 39x supports per CPU interrupts for gpios, like Armada XP.

So add compatible string "marvell,armadaxp-gpio" for Armada 39x GPIO nodes.

Driver gpio-mvebu.c which handles both pre-XP and XP variants already
provides support for per CPU interrupts on XP and newer variants.

Signed-off-by: Pali Rohár <pali@kernel.org>
Fixes: d81a914f ("ARM: dts: mvebu: armada-39x: add missing nodes describing GPIO's")
Signed-off-by: Gregory CLEMENT <gregory.clement@bootlin.com>