dt-bindings: memory-controllers: ti,gpmc: Convert to yaml

Convert omap-gpmc.txt to ti,gpmc.yaml.

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>