MAINTAINERS: Add me as maintainer of Nokia N900 power supply drivers

I'm author of two of those drivers (bq2415x and rx51) and I added
more patches for other twos. Also I fished (proper) open source
power management software for Nokia N900 and those kernel drivers
are part of it.

Signed-off-by: Pali Rohár <pali.rohar@gmail.com>
Signed-off-by: Sebastian Reichel <sre@kernel.org>