dt-bindings: nvmem: u-boot,env: add basic NVMEM cells

U-Boot doesn't have cells at hardcoded addresses. They are stored in
internal 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.
Signed-off-by: Rafał Miłecki <rafal@milecki.pl>
Reviewed-by: Tom Rini <trini@konsulko.com>
Link: https://lore.kernel.org/r/20220703084843.21922-1-zajec5@gmail.comSigned-off-by: Rob Herring <robh@kernel.org>

Documentation/devicetree/bindings/nvmem/u-boot,env.yaml

@@ -24,6 +24,8 @@ description: |
   Right now only flash partition case is covered but it may be extended to e.g.
   UBI volumes in the future.
 
+  Variables can be defined as NVMEM device subnodes.
+
 maintainers:
   - Rafał Miłecki <rafal@milecki.pl>
 
@@ -40,6 +42,14 @@ properties:
   reg:
     maxItems: 1
 
+  bootcmd:
+    type: object
+    description: Command to use for automatic booting
+
+  ethaddr:
+    type: object
+    description: Ethernet interface's MAC address
+
 additionalProperties: false
 
 examples:
@@ -58,5 +68,8 @@ examples:
         env: partition@40000 {
             compatible = "u-boot,env";
             reg = <0x40000 0x10000>;
+
+            mac: ethaddr {
+            };
         };
     };