gpio: Add DM GPIO driver for Marvell MVEBU

This patch adds a DM GPIO driver for the Marvell MVEBU SoCs. There are
other non-DM drivers that might be used on these platforms. But this
patch creates a new DM driver. Which will be used by all Armada XP/38x
boards. Other MVEBU SoC (Kirkwood / Orion) may follow once they
support DM as well.

Signed-off-by: Stefan Roese <sr@denx.de>
Cc: Dirk Eibach <dirk.eibach@gdsys.cc>
Cc: Phil Sutter <phil@nwl.cc>
Cc: Kevin Smith <kevin.smith@elecsyscorp.com>
Cc: Luka Perkov <luka.perkov@sartura.hr>
Cc: Tom Rini <trini@konsulko.com>
Reviewed-by: Kevin Smith <kevin.smith@elecsyscorp.com>
Tested-by: Kevin Smith <kevin.smith@elecsyscorp.com>
Signed-off-by: Stefan Roese <sr@denx.de>