ARM64: Remove duplicated Kconfig entry for "kernel/power/Kconfig"

There is a duplicated Kconfig entry for "kernel/power/Kconfig"
in menu "Power management options" and "CPU Power Management",
remove the one from menu "CPU Power Management" suggested by
Viresh.

Signed-off-by: Hanjun Guo <hanjun.guo@linaro.org>
Reviewed-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>