cpuidle: haltpoll: add missing MODULE_DESCRIPTION() macro

make allmodconfig && make W=1 C=1 reports:
WARNING: modpost: missing MODULE_DESCRIPTION() in drivers/cpuidle/cpuidle-haltpoll.o

Add the missing invocation of the MODULE_DESCRIPTION() macro.

Signed-off-by: Jeff Johnson <quic_jjohnson@quicinc.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>