mtd: spi-nor: Stop exporting spi_nor_restore()

Some SPI NOR controllers that used this method were moved to
drivers/spi/. We don't accept new support for the existing SPI NOR
controllers drivers under drivers/mtd/spi-nor/controllers/ and we
encourage their owners to move the drivers under drivers/spi/.
Make spi_nor_restore() private as we're going to use it just in core.c.

Link: https://lore.kernel.org/r/20230331074606.3559258-8-tudor.ambarus@linaro.org
Signed-off-by: Tudor Ambarus <tudor.ambarus@linaro.org>