mtd: docg3: add device-tree support

Add device-tree support. This is straightforward as docg3 only uses the
standard IOMEM resources.

Signed-off-by: Robert Jarzmik <robert.jarzmik@free.fr>
Cc: devicetree@vger.kernel.org
Cc: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Brian Norris <computersforpeace@gmail.com>