powerpc/powernv: Add OPAL message log interface

OPAL provides an in-memory circular buffer containing a message log
populated with various runtime messages produced by the firmware.

Provide a sysfs interface /sys/firmware/opal/msglog for userspace to
view the messages.

Signed-off-by: Joel Stanley <joel@jms.id.au>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>