cpufreq: dt: Update kconfig description

The cpufreq DT driver also supports systems that have multiple
clock/voltage domains for CPUs, i.e. multiple policy systems.

The description of the Kconfig entry was never updated after the driver
was modified to support such systems, fix it.

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