Maintainers.txt: Update maintainers for Marvell platforms

Marcin and Narinder are the people most familiar with the Marvell
platforms, so make them the maintainers.
I move myself down to Reviewer for now.

Cc: Marcin Wojtas <marcin.s.wojtas@gmail.com>
Cc: Narinder Dhillon <ndhillon@marvell.com>
Cc: Michael D Kinney <michael.d.kinney@intel.com>
Signed-off-by: Leif Lindholm <quic_llindhol@quicinc.com>
Reviewed-by: Michael D Kinney <michael.d.kinney@intel.com>
Reviewed-by: Marcin Wojtas <marcin.s.wojtas@gmail.com>
Reviewed-by: Narinder Dhillon <ndhillon@marvell.com>