lisa.energy_meter: Use SimpleMultiSrcConf for conf classes
lisa.energy_meter: Use SimpleMultiSrcConf for conf classes

Mirror what is done in TargetConf. Avoid having an extra "conf" level in
the expected YAML format, which is useless in this kind of
configuration.

ARM-software/lisa,credp/lisa,credp/lisa,credp/lisa,credp/lisa,ARM-software/lisa,ARM-software/lisa,ARM-software/lisa