xics: Implement H_IPOLL

xics: Implement H_IPOLL

This adds support for the H_IPOLL hypercall which the guest
uses to poll for a pending interrupt. This hypercall is
mandatory for PAPR+ and there is no way for the guest to
detect whether it is supported or not so just add it.

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Alexey Kardashevskiy <aik@ozlabs.ru>
Acked-by: Alexander Graf <agraf@suse.de>
Signed-off-by: Alexander Graf <agraf@suse.de>