Documentation: DT: Add LTC2952 poweroff bindings

LTC2952 is an external power control chip, which signals
the OS to shut down. This patch documents the DT binding
for the chip.

Signed-off-by: René Moll <rene.moll@xsens.com>
Signed-off-by: Sebastian Reichel <sre@kernel.org>

diff --git a/Documentation/devicetree/bindings/power/reset/ltc2952-poweroff.txt b/Documentation/devicetree/bindings/power/reset/ltc2952-poweroff.txt
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+Binding for the LTC2952 PowerPath controller
+
+This chip is used to externally trigger a system shut down. Once the trigger has
+been sent, the chips' watchdog has to be reset to gracefully shut down.
+If the Linux systems decides to shut down it powers off the platform via the
+kill signal.
+
+Required properties:
+
+- compatible:          Must contain: "lltc,ltc2952"
+- trigger-gpios:       phandle + gpio-specifier for the GPIO connected to the
+                       chip's trigger line
+- watchdog-gpios:      phandle + gpio-specifier for the GPIO connected to the
+                       chip's watchdog line
+- kill-gpios:          phandle + gpio-specifier for the GPIO connected to the
+                       chip's kill line
+
+Example:
+
+ltc2952 {
+       compatible = "lltc,ltc2952";
+
+       trigger-gpios = <&gpio0 1 GPIO_ACTIVE_LOW>;
+       watchdog-gpios = <&gpio1 2 GPIO_ACTIVE_HIGH>;
+       kill-gpios = <&gpio0 2 GPIO_ACTIVE_LOW>;
+};