MAINTAINERS: update entry for Marvell ARM platform maintainers

The Marvell platform support is no longer limited to Armada 370, 375,
38x and XP, but now also includes the 32-bits Armada 39x and the
64-bits Armada 3700 and 7K/8K.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Gregory CLEMENT <gregory.clement@free-electrons.com>