MAINTAINERS: Add entry for ATC260x PMIC

Signed-off-by: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>
[cristian: change binding doc file path, add file patterns for onkey and
           poweroff drivers, fix ordering, add myself as co-maintainer]
Signed-off-by: Cristian Ciocaltea <cristian.ciocaltea@gmail.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>