Add a schema for the NVMEM layout on Kontron's sl28 boards.

Signed-off-by: Michael Walle <redacted>
---
changes since v1:
 - add custom select
 - add description
 - add "additionalProperties: false", I wasn't sure if all the
   subnodes needs it. I'd say yes, but the brcm,nvram binding
   doesn't have them neither.

 .../nvmem/layouts/kontron,sl28-vpd.yaml       | 67 +++++++++++++++++++
 1 file changed, 67 insertions(+)
 create mode 100644 Documentation/devicetree/bindings/nvmem/layouts/kontron,sl28-vpd.yaml

diff --git a/Documentation/devicetree/bindings/nvmem/layouts/kontron,sl28-vpd.yaml b/Documentation/devicetree/bindings/nvmem/layouts/kontron,sl28-vpd.yaml
new file mode 100644
index 000000000000..0c180f29e880
--- /dev/null
+++ b/Documentation/devicetree/bindings/nvmem/layouts/kontron,sl28-vpd.yaml

@@ -0,0 +1,67 @@
+# SPDX-License-Identifier: (GPL-2.0 OR BSD-2-Clause)
+%YAML 1.2
+---
+$id: http://devicetree.org/schemas/nvmem/layouts/kontron,sl28-vpd.yaml#
+$schema: http://devicetree.org/meta-schemas/core.yaml#
+
+title: NVMEM layout of the Kontron SMARC-sAL28 vital product data
+
+maintainers:
+  - Michael Walle <michael@walle.cc>
+
+description:
+  The vital product data (VPD) of the sl28 boards contains a serial
+  number and a base MAC address. The actual MAC addresses for the
+  on-board ethernet devices are derived from this base MAC address by
+  adding an offset.
+
+# We need a select here so we don't match all nodes with 'user-otp'
+select:
+  properties:
+    compatible:
+      contains:
+        const: kontron,sl28-vpd
+  required:
+    - compatible
+
+properties:
+  compatible:
+    items:
+      - const: kontron,sl28-vpd
+      - const: user-otp
+
+  serial-number:
+    type: object
+    description: The board's serial number
+
+  base-mac-address:
+    type: object
+    description:
+      Base MAC address for all on-module network interfaces. The first
+      argument of the phandle will be treated as an offset.
+
+    properties:
+      "#nvmem-cell-cells":