hmi: Add locking to hmi handler
hmi: Add locking to hmi handler

Take a lock before handle_hmi_event per Ben's suggestion.  So, when we clear
events, only one thread per core will report it.

Signed-off-by: Ryan Grimm <b82eecdfce19ef6c0125840b498d3afc6226dd31@linux.vnet.ibm.com>
Signed-off-by: Benjamin Herrenschmidt <a7089bb6e7e92505d88aaff006cbdd60cc9120b6@kernel.crashing.org>

mikey/skiboot,stewart-ibm/skiboot,qemu/skiboot,open-power/skiboot,open-power/skiboot,mikey/skiboot,stewart-ibm/skiboot,legoater/skiboot,legoater/skiboot,mikey/skiboot,legoater/skiboot,cyrilbur-ibm/skiboot,csmart/skiboot,cyrilbur-ibm/skiboot,open-power/skiboot,jk-ozlabs/skiboot,neelegup/skiboot,open-power/skiboot,neelegup/skiboot,shenki/skiboot,qemu/skiboot,cyrilbur-ibm/skiboot,legoater/skiboot,stewart-ibm/skiboot,ddstreet/skiboot,qemu/skiboot,csmart/skiboot,jk-ozlabs/skiboot,csmart/skiboot,shenki/skiboot,open-power/skiboot,shenki/skiboot,shenki/skiboot,neelegup/skiboot,apopple/skiboot,legoater/skiboot,apopple/skiboot,apopple/skiboot,jk-ozlabs/skiboot,qemu/skiboot,ddstreet/skiboot,qemu/skiboot,ddstreet/skiboot,shenki/skiboot