dt/bindings: memory: Add bindings for TI GPMC driver

GPMC stands for General Purpose Memory Controller and it is
present on many Texas Instruments SoCs.

It supports a number of Asynchronous and Synchronous interfaces
and has various settings to configure the bus interface.

The DT bindings define all the various GPMC settings.

As the GPMC supports multiple devices on the bus, each
device is represented as a child and the respective
GPMC settings are situated there. (see ti,gpmc-child.yaml)

These binding docs are picked up from the Linux kernel.

Signed-off-by: Roger Quadros <rogerq@kernel.org>