MAINTAINERS: add the Renesas X9250 driver entry

After contributing the driver, add myself as the maintainer for the
Renesas X9250 IIO driver.

Signed-off-by: Herve Codina <herve.codina@bootlin.com>
Link: https://lore.kernel.org/r/20230509160852.158101-4-herve.codina@bootlin.com


Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>