ANDROID: sched/fair: Change cpu iteration order in find_best_target()

The schedtune task parameter 'boosted' is mapped into the cpu iteration
order. Currently for 'boosted' equal true the iteration starts at the
last cpu (NR_CPUS-1) whereas for 'boosted' equal false it starts at the
first cpu (0).

This only has the desired effect if the cpu topology oerdering matches
the underlying assumption. This e.g. is the case for the
Qc snapdragon 821 with its [L0 L1 b0 b1] cpu topology layout
(L=lower max freq, b=higher max freq). This results in cpus with higher
maximum capacity being given the highest logical cpu ids. However not
all big.LITTLE systems enumerate their cpus in the same way. For example,
the ARM Versatile Express Juno board has 6 cpus for which the default
configuration has topology [L0 b0 b1 L1 L2 L3].

To make this approach independent from the cpu topology layout it now
iterates over the cpus in the order of the sched_groups of the EAS
sched_domain (sd_ea). The order of cpu iteration is different for the
different cpu types in case the cpu is used to dereference sd_ea.

Considering the Qc snapdragon 821 again, for cpu L0 and L1 the order is
'b0->b1->L0->L1' whereas for b0 and b1 the order is 'b0->b1->L0->L1'.

This approach does not allow the exact same iteration order as with the
currently used flat iteration over [0 .. NR_CPUS-1] but the cpus
are ordered by the original cpu capacity.

The cpu iteration is now done in the sd_ea sched_group order required by
the 'boosted' value ['L0->L1->b0->b1'/'b0->b1->L0->L1'] rather than
forward/backward over the flat cpu space ['L0->L1->b0->b1'/
'b1->b0->L1->L0'].

Change-Id: I8fbe2073dedd2ecb1c750620c6000c11a5ff4358
Signed-off-by: Dietmar Eggemann <dietmar.eggemann@arm.com>
(cherry picked from commit a0c6a4272c3968c0ff50d3fed65f5865b72d777b)
Signed-off-by: Chris Redpath <chris.redpath@arm.com>
Signed-off-by: Quentin Perret <quentin.perret@arm.com>