powerpc/powernv: Use common code for the symbol_map export

Long before we had a generic way for firmware to export memory ranges of
interest we added a special case for the skiboot symbol map. The code is
pretty much identical to the generic export so re-use the code.

Signed-off-by: Oliver O'Halloran <oohall@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20191101062611.32610-2-oohall@gmail.com