cpufreq: Select schedutil when using big.LITTLE

When we are using a system with big.LITTLE HMP
configuration, we need to use EAS to schedule the
system.

As can be seen from kernel/sched/topology.c:

 "EAS can be used on a root domain if it meets all the following conditions:
  1. an Energy Model (EM) is available;
  2. the SD_ASYM_CPUCAPACITY flag is set in the sched_domain hierarchy.
  3. no SMT is detected.
  4. the EM complexity is low enough to keep scheduling overheads low;
  5. schedutil is driving the frequency of all CPUs of the rd;"

This means that at the very least, schedutil needs to be
available as a scheduling policy for EAS to work on these
systems. Make this explicit by defaulting to the schedutil
governor if BIG_LITTLE is selected.

Currently users of the TC2 board (like me) has to figure these
dependencies out themselves and it is not helpful.

Suggested-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Acked-by: Sudeep Holla <sudeep.holla@arm.com>
Acked-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>