cpuidle-haltpoll: condition on ARCH_CPUIDLE_HALTPOLL

cpuidle-haltpoll: condition on ARCH_CPUIDLE_HALTPOLL

The cpuidle-haltpoll driver and its namesake governor are selected
under KVM_GUEST on X86. KVM_GUEST in-turn selects ARCH_CPUIDLE_HALTPOLL
and defines the requisite arch_haltpoll_{enable,disable}() functions.

So remove the explicit dependence of HALTPOLL_CPUIDLE on KVM_GUEST,
and instead use ARCH_CPUIDLE_HALTPOLL as proxy for architectural
support for haltpoll.

Also change "halt poll" to "haltpoll" in one of the summary clauses,
since the second form is used everywhere else.

Signed-off-by: Ankur Arora <ankur.a.arora@oracle.com>
Acked-by: Will Deacon <will@kernel.org>
Tested-by: Misono Tomohiro <misono.tomohiro@fujitsu.com>