powerpc/powernv: Add OPAL exports attributes to sysfs

New versions of OPAL have a device node /ibm,opal/firmware/exports, each
property of which describes a range of memory in OPAL that Linux might
want to export to userspace for debugging.

This patch adds a sysfs file under 'opal/exports' for each property
found there, and makes it read-only by root.

Signed-off-by: Matt Brown <matthew.brown.dev@gmail.com>
[mpe: Drop counting of props, rename to attr, free on sysfs error, c'log]
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>