dt-bindings: memory-controllers: ti,gpmc: add wait-pin polarity

dt-bindings: memory-controllers: ti,gpmc: add wait-pin polarity

The GPMC controller has the ability to configure the polarity for the
wait pin. The current properties do not allow this configuration.
This binding directly configures the WAITPIN<X>POLARITY bit
in the GPMC_CONFIG register by setting the "ti,wait-pin-polarity"
dt-property.

Signed-off-by: Benedikt Niedermayr <benedikt.niedermayr@siemens.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Reviewed-by: Roger Quadros <rogerq@kernel.org>
Link: https://lore.kernel.org/r/20221102133047.1654449-3-benedikt.niedermayr@siemens.com
Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>