rtc: mt6397: expose the spare bytes of the alarm registers as nvmem

Each alarm field lives in the low bits of its register and the driver
masks its writes accordingly, so the high byte of four of them is storage
the RTC never touches. MediaTek names these RTC_NEW_SPARE0 to
RTC_NEW_SPARE3 and gives the first to a fuel gauge, which is how its PMIC
battery drivers carry a state of charge over a reboot.

Offer all four as a battery-backed nvmem provider, so that a consumer does
not have to reach into this block behind the driver's back. Doing it here
is what makes it safe: a write lands under the same lock the alarm paths
take, so it can neither be lost inside mtk_rtc_set_alarm()'s
read-modify-write nor fire the write trigger in the middle of one.

The nvmem core does not range check a cell against the provider size, so
the callbacks check the offset themselves.

Tested on an MT6397; MediaTek's spare map for mt6323 matches, and the
alarm field masks are common to every compatible this driver binds.

Assisted-by: LLM
Signed-off-by: Ryan Brue <ryanbrue.dev@gmail.com>
Reviewed-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Link: https://patch.msgid.link/20260917-rbrue-suez-upstreaming-mt6397-rtc-nvmem-v1-2-558b8f95cfbc@gmail.com
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>

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