PowerNV: add buffer for OPAL console

Avoid the lock in vtophys() by providing a static direct-mapped
spinlock- protected output buffer to use when the console driver
cannot acquire locks for some reason. This allows the idle thread
to use printf() (e.g. the SMP startup messages) without crashing
the kernel.

Created by: Nathan Whitehorn <nwhitehorn@freebsd.org>
Submitted by: Wojciech Macek <wma@freebsd.org>
Sponsored by: FreeBSD Foundation