cpuidle: clean up dead dependencies on CPU_IDLE in Kconfig

cpuidle: clean up dead dependencies on CPU_IDLE in Kconfig

The Kconfig in the parent directory already has the first 'if CPU_IDLE'
gating the inclusion of this Kconfig, meaning that the 'depends on
CPUIDLE' statements in these config options are effectively dead code.

Leave the 'if CPU_IDLE...endif' condition, and remove the individual
'depends on' statements in Kconfig.mips and Kconfig.powerpc

This dead code was found by kconfirm, a static analysis tool for
Kconfig.

Signed-off-by: Julian Braha <julianbraha@gmail.com>
[ rjw: Subject and changelog edits ]
Link: https://patch.msgid.link/20260331074920.41269-1-julianbraha@gmail.com
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>