powerpc/powernv: Rework exports to support subnodes

Originally we only had a handful of exported memory ranges, but we'd to
export the per-core trace buffers. This results in a lot of files in the
exports directory which is a but unfortunate. We can clean things up a bit
by turning subnodes into subdirectories of the exports directory.

Signed-off-by: Oliver O'Halloran <oohall@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20191101062611.32610-1-oohall@gmail.com