rtc: ds3232: get SRAM access using NVMEM Framework

DS3232 RTC has 236 bytes of persistent memory.

Add RTC SRAM read and write access using
the NVMEM Framework.

Signed-off-by: Han Nandor <nandor.han@vaisala.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>