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

Documentation/devicetree/bindings/power/reset/ltc2952-poweroff.txt

+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>;
+};