powerpc/powernv: Create opal sysfs directory

Create /sys/firmware/opal directory. We wil use this
interface to fetch opal error logs, firmware update, etc.

Signed-off-by: Vasant Hegde <hegdevasant@linux.vnet.ibm.com>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>