Documentation: tpm: add the Physical TPM device tree binding documentation

Newly added support of TPM 2.0 eventlog securityfs pseudo files in tpm
device driver consumes device tree bindings representing I2C based
Physical TPM. This patch adds the documentation for corresponding device
tree bindings of I2C based Physical TPM. These bindings are similar to
vtpm device tree bindings being used on IBM Power7+ and Power8 Systems
running PowerVM.
Suggested-by: NJason Gunthorpe <jgunthorpe@obsidianresearch.com>
Signed-off-by: NNayna Jain <nayna@linux.vnet.ibm.com>
Acked-by: NRob Herring <robh@kernel.org>
Signed-off-by: NJarkko Sakkinen <jarkko.sakkinen@linux.intel.com>

Documentation/devicetree/bindings/security/tpm/tpm-i2c.txt

+* Device Tree Bindings for I2C based Trusted Platform Module(TPM)
+
+Required properties:
+
+- compatible     : 'manufacturer,model', eg. nuvoton,npct650
+- label          : human readable string describing the device, eg. "tpm"
+- linux,sml-base : 64-bit base address of the reserved memory allocated for
+                   the firmware event log
+- linux,sml-size : size of the memory allocated for the firmware event log
+
+Example (for OpenPower Systems with Nuvoton TPM 2.0 on I2C)
+----------------------------------------------------------
+
+tpm@57 {
+	reg = <0x57>;
+	label = "tpm";
+	compatible = "nuvoton,npct650", "nuvoton,npct601";
+	linux,sml-base = <0x7f 0xfd450000>;
+	linux,sml-size = <0x10000>;
+	status = "okay";
+};