Documentation: tpm: add powered-while-suspended binding documentation

Add a new powered-while-suspended property to control the behavior of the
TPM suspend/resume.

Signed-off-by: Enric Balletbo i Serra <enric.balletbo@collabora.com>
Signed-off-by: Sonny Rao <sonnyrao@chromium.org>
Reviewed-by: Jason Gunthorpe <jgunthorpe@obsidianresearch.com>
Reviewed-by: Jarkko Sakkinen <jarkko.sakkinen@linux.intel.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Jarkko Sakkinen <jarkko.sakkinen@linux.intel.com>
Signed-off-by: James Morris <james.l.morris@oracle.com>

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

@@ -8,6 +8,12 @@ Required properties:
                    the firmware event log
 - linux,sml-size : size of the memory allocated for the firmware event log
 
+Optional properties:
+
+- powered-while-suspended: present when the TPM is left powered on between
+                           suspend and resume (makes the suspend/resume
+                           callbacks do nothing).
+
 Example (for OpenPower Systems with Nuvoton TPM 2.0 on I2C)
 ----------------------------------------------------------