arm: mvebu: enable gpio expander over i2c on Mirabox platform

The Globalscale Mirabox platform can be connected to the JTAG/GPIO box
through the Multi-IO port. The GPIO box use the NXP PCA9505 I/O port
expansion IC to provide 40-bit parallel input/output GPIOs. This patch
enable the use of this expander on the Mirabox.

Signed-off-by: Gregory CLEMENT <gregory.clement@free-electrons.com>
Acked-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Jason Cooper <jason@lakedaemon.net>