dt-bindings: bus: ti-gpmc: Add AAD timings properties

In order to support advanced AAD timings, add these properties to the DT
GPMC bindings.

Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>
Acked-by: Tony Lindgren <tony@atomide.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Roger Quadros <rogerq@ti.com>