MAINTAINERS: Update u-boot-marvell entry

MAINTAINERS: Update u-boot-marvell entry

This patch does the following changes to the u-boot-marvell maintainers
entry:

- Add Armada-7k/8k to the list
- Remove Prafulla and Luka since they have been silent on the list for
  a long time. Please speak up, if you would like to continue or better
  start maintaining.
- Add multiple Marvell / MVEBU related driver directories and files

Signed-off-by: Stefan Roese <sr@denx.de>
Cc: Prafulla Wadaskar <prafulla@marvell.com>
Cc: Luka Perkov <luka.perkov@sartura.hr>
Cc: Tom Rini <trini@konsulko.com>
Acked-by: Luka Perkov <luka.perkov@sartura.hr>
Signed-off-by: Stefan Roese <sr@denx.de>