arm64: define hikey620 sys sd energy model

Hi6220 has a single frequency domain spanning the two clusters. It
needs the SYS sched domain (sd) to let the EAS algorithm work
properly.

The SD_SHARE_CAP_STATES flag is not set on SYS sd.

This lets sd_ea (highest sd w/ energy model data) point to the SYS
sd whereas sd_scs (highest sd w/ SD_SHARE_CAP_STATES set) points to
the DIE sd. This setup allows the code in sched_group_energy() to
set sg_shared_cap to the single sched group of the SYS sd covering
all the cpus in the system as they are all part of the single
frequency domain.

The capacity and idle state vectors only contain entries w/ power
values equal zero, so there is no system-wide energy contribution.

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