dt-bindings: chosen: Add "power-state-change-reason" nvmem property

A lot of systems (especially embedded) store the last shutdown/reboot
reason in NVMEM. This could be anything from "over temperature" or
"brown-out" to an "EC panic" or watchdog reset.

There's currently no standard way for firmware to tell the kernel where
to find this information.

The /chosen node is already used for passing global, firmware-provided
data like 'bootargs' and 'stdout-path'. Let's use it for this, too.

This patch adds a binding for a new generic NVMEM cell name in /chosen:
"power-state-change-reason".

Example from the binding:
    chosen {
        nvmem-cells = <&pscr_cell>;
        nvmem-cell-names = "power-state-change-reason";
    };

Signed-off-by: Oleksij Rempel <[email protected]>

Author: olerem

dtschema/schemas/chosen.yaml

@@ -167,6 +167,18 @@ properties:
       will assign devices in its usual manner, otherwise it will not try to
       assign devices and instead use them as they are configured already.
 
+  nvmem-cells:
+    maxItems: 1
+    description:
+      A phandle to an NVMEM cell providing system-wide information.
+      Used in conjunction with 'nvmem-cell-names'.
+
+  nvmem-cell-names:
+    items:
+      - const: power-state-change-reason
+    description:
+      Indicates the NVMEM cell contains the last power state change reason
+
   stdout-path:
     $ref: types.yaml#/definitions/string
     pattern: "^[a-zA-Z0-9@/,+\\-._]*(:[0-9]*[noe]?[78]?[r]?)?$"