Add LastRebootTime property to BMC state

It contains the epoch time of the last BMC reboot
as determined by its uptime.

Change-Id: I2e80734db8443c9e3d49b3cf6dd336ee5d9937be
Signed-off-by: Matt Spinler <spinler@us.ibm.com>
diff --git a/xyz/openbmc_project/State/BMC.interface.yaml b/xyz/openbmc_project/State/BMC.interface.yaml
index 45dbc48..98d61a0 100644
--- a/xyz/openbmc_project/State/BMC.interface.yaml
+++ b/xyz/openbmc_project/State/BMC.interface.yaml
@@ -16,6 +16,14 @@
      description: >
          The current state of the BMC and is a read-only property.
 
+   - name: LastRebootTime
+     type: uint64
+     description: >
+        The last time at which the BMC came out of a reboot as
+        determined by its uptime, in epoch time, in milliseconds.
+     errors:
+        - xyz.openbmc_project.Common.Error.InternalFailure
+
 enumerations:
    - name: Transition
      description: >