MAINTAINERS: Update ARM Versatile Express platform entry

This patch update ARM Versatile Express entry to cover bits needed for
Cortex-M Prototyping System (MPS2 platform). So patches for the latter
are collected by existing Vexpress and Juno maintainers (Liviu, Sudeep,
Lorenzo) and find their way upstream via Vexpess and/or Juno trees.

Signed-off-by: Vladimir Murzin <vladimir.murzin@arm.com>
Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>