libs/utils/energy.py: Streamline self.readings calc in HWMon::sample()
libs/utils/energy.py: Streamline self.readings calc in HWMon::sample()

Tested with JUNO target.

Signed-off-by: Dietmar Eggemann <b09ca079358248c3ff5bd91ea3781c6808cc6cf2@arm.com>

ARM-software/lisa,ARM-software/lisa,joelagnel/lisa,ARM-software/lisa,arnoldlu/lisa,credp/lisa,joelagnel/lisa,bjackman/lisa,ARM-software/lisa,credp/lisa,mdigiorgio/lisa,arnoldlu/lisa,JaviMerino/lisa,bjackman/lisa,credp/lisa,mdigiorgio/lisa,credp/lisa