MAINTAINERS: add Marvell MBus driver to Marvell EBU SoCs support

While doing some code cleanup in drivers/bus/, I noticed that the file
drivers/bus/mvebu-mbus.c has no maintainer.

Although the file has not been touched a lot lately, the git history tells
us that Gregory Clement and Andrew Lunn integrated patches specific to
this driver code. Further, the driver's config depends on config
PLAT_ORION, and the code for this platform is defined in
arch/arm/plat-orion/, which is part of ARM/Marvell Dove/MV78xx0/Orion SOC
support with Gregory and Andrew already being its maintainer.

Add drivers/bus/mvebu-mbus.c to ARM/Marvell Dove/MV78xx0/Orion SOC support.

Signed-off-by: Lukas Bulwahn <lukas.bulwahn@gmail.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Gregory CLEMENT <gregory.clement@bootlin.com>