gpio: mvebu: allow building driver for Armada 7K/8K

The mvebu gpio driver can also be used on arm64 mvebu SoC such as the
Armada 7K/8K. This commit allows to build the driver for them (when only
ARCH_MVEBU is defined)

Reviewed-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Gregory CLEMENT <gregory.clement@free-electrons.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>