MAINTAINERS: make me the maintainer of DaVinci platforms

Sekhar is stepping down from supporting DaVinci. As it's quite low-volume,
I will keep maintaining it.

Acked-by: Sekhar Nori <nsekhar@ti.com>
Acked-by: Kevin Hilman <khilman@baylibre.com>
Signed-off-by: Bartosz Golaszewski <brgl@bgdev.pl>