arm: mvebu: add DT information for GPIO banks on Armada 370 and XP

The gpioX aliases are needed so that the driver can use
of_alias_get_id() to get a 0-based number of the GPIO bank, which we
then use to compute the base GPIO of the bank being probed. This is
similar to what gpio-mxs.c is doing.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Cc: Jason Cooper <jason@lakedaemon.net>
Cc: Andrew Lunn <andrew@lunn.ch>
Cc: Gregory Clement <gregory.clement@free-electrons.com>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Jason Cooper <jason@lakedaemon.net>