MAINTAINERS: drivers/power: add entry for SmartReflex AVS drivers

The SmartReflex AVS driver evolved out of the OMAP kernel and now lives
under drivers/power/avs.

I've historically been maintainer of this but Nishanth Menon is doing most
of the heavy lifting now.  Add us both as co-maintainers.

Cc: David Woodhouse <dwmw2@infradead.org>
Cc: Rafael J. Wysocki <rjw@sisk.pl>
Cc: Nishanth Menon <nm@ti.com>
Signed-off-by: Kevin Hilman <khilman@linaro.org>
Signed-off-by: Anton Vorontsov <anton@enomsg.org>