tpm/tpm_i2c_stm_st33/dts/st33zp24_i2c: Add DTS Documentation

st33zp24 tpm can be seen as a trivial i2c device as other i2c tpm.
However several other properties needs to be documented such as lpcpd.
Reviewed-by: Jason Gunthorpe <jgunthorpe@obsidianresearch.com>
Signed-off-by: Christophe Ricard <christophe-h.ricard@st.com>
Signed-off-by: Peter Huewe <peterhuewe@gmx.de>

Documentation/devicetree/bindings/security/tpm/st33zp24.txt

+* STMicroelectronics SAS. ST33ZP24 TPM SoC
+
+Required properties:
+- compatible: Should be "st,st33zp24-i2c".
+- clock-frequency: I²C work frequency.
+- reg: address on the bus
+
+Optional ST33ZP24 Properties:
+- interrupt-parent: phandle for the interrupt gpio controller
+- interrupts: GPIO interrupt to which the chip is connected
+- lpcpd-gpios: Output GPIO pin used for ST33ZP24 power management D1/D2 state.
+If set, power must be present when the platform is going into sleep/hibernate mode.
+
+Optional SoC Specific Properties:
+- pinctrl-names: Contains only one value - "default".
+- pintctrl-0: Specifies the pin control groups used for this controller.
+
+Example (for ARM-based BeagleBoard xM with ST33ZP24 on I2C2):
+
+&i2c2 {
+
+        status = "okay";
+
+        st33zp24: st33zp24@13 {
+
+                compatible = "st,st33zp24-i2c";
+
+                reg = <0x013>;
+                clock-frequency = <400000>;
+
+                interrupt-parent = <&gpio5>;
+                interrupts = <7 IRQ_TYPE_LEVEL_HIGH>;
+
+                lpcpd-gpios = <&gpio5 15 GPIO_ACTIVE_HIGH>;
+        };
+};