dt-bindings: mfd: mediatek: mt6397: describe the RTC's nvmem layout

Four of the MT6397 RTC's alarm registers use only their low bits; the
high byte of each is storage the clock and the alarm don't touch.
MediaTek's documentation names them RTC_NEW_SPARE0 to RTC_NEW_SPARE3 and
assigns the first to a fuel gauge, which is what a battery driver reads
at boot so that the reported capacity does not jump across a reboot.

They sit in the RTC's always-on domain, so the driver can offer them as
a battery-backed nvmem provider. Allow a board to lay cells out over
them.

Assisted-by: LLM
Signed-off-by: Ryan Brue <ryanbrue.dev@gmail.com>
Reviewed-by: Rob Herring (Arm) <robh@kernel.org>
Reviewed-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Link: https://patch.msgid.link/20260917-rbrue-suez-upstreaming-mt6397-rtc-nvmem-v1-1-558b8f95cfbc@gmail.com
Signed-off-by: Lee Jones <lee@kernel.org>

Documentation/devicetree/bindings/mfd/mediatek,mt6397.yaml

@@ -84,6 +84,15 @@ properties:
 
       start-year: true
 
+      nvmem-layout:
+        $ref: /schemas/nvmem/layouts/nvmem-layout.yaml
+        description:
+          The RTC carries four bytes of storage that neither the clock nor the
+          alarm uses, in the high half of four of the alarm registers, and
+          offers them as a battery-backed nvmem provider. MediaTek's
+          documentation names them RTC_NEW_SPARE0 to RTC_NEW_SPARE3 and gives
+          the first to a fuel gauge.
+
     required:
       - compatible