cpufreq: Fix typo in the ARM_BRCMSTB_AVS_CPUFREQ Kconfig entry

Delete the redundant word 'to' from the help text in the
ARM_BRCMSTB_AVS_CPUFREQ Kconfig entry.

Signed-off-by: Jingyu Wang <jingyuwang_vip@163.com>
[ rjw: Subject and changelog edits ]
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>