cpufreq: blackfin: enable driver for CONFIG_BFIN_CPU_FREQ

By mistake blackfin's cpufreq driver is enabled when CONFIG_BLACKFIN was
present, whereas it should have been enabled only when CONFIG_BFIN_CPU_FREQ is
present.

Fix it.

Acked-by: Mike Frysinger <vapier@gentoo.org>
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>