ARM: dts: berlin: add GPIO nodes for the BG2Q

The Marvell Berlin BG2Q has 6 GPIO ports compatible with the snps,dw-apb-gpio
driver. This patch adds the corresponding device tree nodes.

Signed-off-by: Antoine Ténart <antoine.tenart@free-electrons.com>
Reviewed-by: Jisheng Zhang <jszhang@marvell.com>
Signed-off-by: Sebastian Hesselbarth <sebastian.hesselbarth@gmail.com>