cpufreq: remove tango driver

The tango platform is getting removed, so the driver is no
longer needed.

Cc: Marc Gonzalez <marc.w.gonzalez@free.fr>
Cc: Mans Rullgard <mans@mansr.com>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
[ Viresh: Update cpufreq-dt-platdev.c as well ]
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>