dt-bindings: wkup-m3-ipc: Add vtt toggle gpio pin property

Document Wakeup M3 IPC property that indicates a GPIO pin is connected
to the enable pin on DDR VTT regulator and can be toggled during low
power mode transitions.

Signed-off-by: Dave Gerlach <d-gerlach@ti.com>
[dfustini: converted to YAML, removed unnecessary "ti,needs-vtt-toggle"]
Signed-off-by: Drew Fustini <dfustini@baylibre.com>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20220409211215.2529387-2-dfustini@baylibre.com