dt-bindings: nvmem: brcm,nvram: add basic NVMEM cells

NVRAM doesn't have cells at hardcoded addresses. They are stored in
internal struct (custom & dynamic format). It's still important to
define relevant cells in DT so NVMEM consumers can reference them.

Update binding to allow including basic cells as NVMEM device subnodes.

Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Rafał Miłecki <rafal@milecki.pl>
Signed-off-by: Srinivas Kandagatla <srinivas.kandagatla@linaro.org>
Link: https://lore.kernel.org/r/20220225175822.8293-5-srinivas.kandagatla@linaro.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Documentation/devicetree/bindings/nvmem/brcm,nvram.yaml

@@ -14,6 +14,8 @@ description: |
   NVRAM can be accessed on Broadcom BCM47xx MIPS and Northstar ARM Cortex-A9
   devices usiong I/O mapped memory.
 
+  NVRAM variables can be defined as NVMEM device subnodes.
+
 maintainers:
   - Rafał Miłecki <rafal@milecki.pl>
 
@@ -27,11 +29,30 @@ properties:
   reg:
     maxItems: 1
 
+  board_id:
+    type: object
+    description: Board identification name
+
+  et0macaddr:
+    type: object
+    description: First Ethernet interface's MAC address
+
+  et1macaddr:
+    type: object
+    description: Second Ethernet interface's MAC address
+
+  et2macaddr:
+    type: object
+    description: Third Ethernet interface's MAC address
+
 unevaluatedProperties: false
 
 examples:
   - |
     nvram@1eff0000 {
-            compatible = "brcm,nvram";
-            reg = <0x1eff0000 0x10000>;
+        compatible = "brcm,nvram";
+        reg = <0x1eff0000 0x10000>;
+
+        mac: et0macaddr {
+        };
     };