mtd: docg3: add device-tree documentation

Add documentation for the sandisk docg3 chip.

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>