cpufreq: boost: Remove CONFIG_CPU_FREQ_BOOST_SW Kconfig option

Commit 2fb4719b ("cpufreq / boost: Kconfig: Support for
software-managed BOOST") added the CONFIG_CPU_FREQ_BOOST_SW config.
However EXYNOS based cpufreq drivers have been removed because of
switching to cpufreq-dt driver which will set boost-attr if required.

So, let's remove this option and update cpufreq_generic_attr[].

Signed-off-by: Yue Hu <huyue2@yulong.com>
Acked-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>