MAINTAINERS: Add maintainers for VME subsystem

Add Martyn Welch, Manohar Vanga and Greg Kroah-Hartman as maintainers for
the VME subsystem.

Signed-off-by: Martyn Welch <martyn.welch@ge.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>