powerpc/powernv: Invoke opal call to handle hmi.

When we hit the HMI in Linux, invoke opal call to handle/recover from HMI
errors in real mode and then in virtual mode during check_irq_replay()
invoke opal_poll_events()/opal_do_notifier() to retrieve HMI event from
OPAL and act accordingly.

Now that we are ready to handle HMI interrupt directly in linux, remove
the HMI interrupt registration with firmware.

Signed-off-by: Mahesh Salgaonkar <mahesh@linux.vnet.ibm.com>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>