energy_model: Fix loading simplified EM from target w/o 'capacity'
energy_model: Fix loading simplified EM from target w/o 'capacity'

The v5 of the simplified EM doesn't expose the 'capacity' file in sysfs.
Fix this by checking if the file exists, and re-compute those values
form within LISA if not.

Signed-off-by: Quentin Perret <2d02971ddd46effd323a33463a71f9ad92a7835e@arm.com>

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