MAINTAINERS: Add myself as maintainer of the RZN1 RTC driver

After contributing it, I'll volunteer to maintain it.

Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Acked-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Link: https://lore.kernel.org/r/20220516082504.33913-6-miquel.raynal@bootlin.com