rtc: pm8xxx: add support for nvmem offset

On many Qualcomm platforms the PMIC RTC control and time registers are
read-only so that the RTC time can not be updated. Instead an offset
needs be stored in some machine-specific non-volatile memory, which the
driver can take into account.

Add support for storing a 32-bit offset from the Epoch in an nvmem cell
so that the RTC time can be set on such platforms.

Signed-off-by: Johan Hovold <johan+linaro@kernel.org>
Link: https://lore.kernel.org/r/20230202155448.6715-18-johan+linaro@kernel.org
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>