dt-bindings: memory-controllers: Introduce ti,gpmc-child

This binding is meant for the child nodes of the TI GPMC node. The node
represents any device connected to the GPMC bus. It may be a Flash chip,
RAM chip or Ethernet controller, etc. These properties are meant for
configuring the GPMC settings/timings and will accompany the bindings
supported by the respective device.

Signed-off-by: Roger Quadros <rogerq@kernel.org>
Reviewed-by: Rob Herring <robh@kernel.org>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@canonical.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>