cpufreq: arm-big-little: Make driver dependent on CONFIG_BIG_LITTLE

The arm_big_little cpufreq driver is only used by ARM bigLITTLE
platforms and hence must depend on CONFIG_BIG_LITTLE.

This was highlighted by Russell earlier when he reported this issue:

drivers/built-in.o: In function `bL_cpufreq_set_rate':
powercap_sys.c:(.text+0x5ed9a0): undefined reference to `bL_switch_request_cb'

Reported-by: Russell King <linux@arm.linux.org.uk>
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>