powerpc/powernv: Check OPAL elog calls exist before using

Check that the OPAL_ELOG_READ token exists before initalising the elog
infrastructure.

This avoids littering the OPAL console with:
  "OPAL: Called with bad token 74"

Signed-off-by: Michael Neuling <mikey@neuling.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>