mtd: mtdoops: Add a timestamp to the mtdoops header.

On some systems, the oops only has relative time from boot.

Signed-off-by: Jean-Marc Eurin <jmeurin@google.com>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/linux-mtd/20220425160927.3823016-1-jmeurin@google.com