BMIPS: Enable prerequisites for CPUfreq in MIPS Kconfig.

Turn on CPU_SUPPORTS_CPUFREQ and MIPS_EXTERNAL_TIMER for BMIPS.

Signed-off-by: Markus Mayer <mmayer@broadcom.com>
Acked-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>