dt-binding: mtd: physmap: Add example using addr-gpios property

Add an example showing how to use the addr-gpios property to deal with a
system with limited IO space.

Cc: devicetree@vger.kernel.org
Signed-off-by: Chris Packham <chris.packham@alliedtelesis.co.nz>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Richard Weinberger <richard@nod.at>