rtc: twl: add NVRAM support

Export SRAM using nvmem.

Signed-off-by: Ladislav Michl <ladis@linux-mips.org>
Reviewed-by: Tony Lindgren <tony@atomide.com>
Link: https://lore.kernel.org/r/ZDf7qZTiml0ijD2g@lenoch


Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>