powernv: Add Hypervisor Maintenance Interrupt handler

Attempting to build www/firefox on POWER9 resulted in a HMI exception being
thrown, a fatal trap currently. This is typically caused by timer facility
errors, but examination of the Hypervisor Maintenance Exception Register
(HMER) yielded only that an exception had recovered, with no information of
the actual exception cause.

When an HMI occurs, OPAL_HANDLE_HMI or OPAL_HANDLE_HMI2 must be called to
handle the exception at the firmware level. If the exception is handled, we
can continue.

This adds only the preliminary handler, enough to prevent package building
from panicking. An enhancement in the future is to use the flags returned
by OPAL_HANDLE_HMI2 to print more useful error messages, and log maintenance
events.

Reviewed by: luporl
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D19634