MAINTAINERS: Add entry for mp2629 Battery Charger driver

Add MAINTAINERS entry for Monolithic Power Systems mp2629 Charger driver.

Signed-off-by: Saravanan Sekar <sravanhome@gmail.com>
Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Reviewed-by: Sebastian Reichel <sebastian.reichel@collabora.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>