dt-bindings: mtd: Add ti, gpmc-nand DT binding documentation

Add DT binding documentation for the TI GPMC NAND controller.
This is picked up from the Linux Kernel.

Signed-off-by: Roger Quadros <rogerq@kernel.org>
Signed-off-by: Michael Trimarchi <michael@amarulasolutions.com>
Signed-off-by: Dario Binacchi <dario.binacchi@amarulasolutions.com>
Link: https://lore.kernel.org/all/20221220102203.52398-4-rogerq@kernel.org