MAINTAINERS: spi-nor: add myself as maintainer

After being a reviewer for a while, add myself as a maintainer for the
spi-nor subsystem.
Signed-off-by: Michael Walle <michael@walle.cc>
Acked-by: Richard Weinberger <richard@nod.at>
Acked-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/r/20231121122134.1952738-1-michael@walle.ccSigned-off-by: Tudor Ambarus <tudor.ambarus@linaro.org>

MAINTAINERS

@@ -20426,7 +20426,7 @@ F:	drivers/pinctrl/spear/
 SPI NOR SUBSYSTEM
 M:	Tudor Ambarus <tudor.ambarus@linaro.org>
 M:	Pratyush Yadav <pratyush@kernel.org>
-R:	Michael Walle <michael@walle.cc>
+M:	Michael Walle <michael@walle.cc>
 L:	linux-mtd@lists.infradead.org
 S:	Maintained
 W:	http://www.linux-mtd.infradead.org/