powerpc/85xx: Add SRIO support to P2020DS

The P2020 has 2 SRIO ports and they are useable on the P2020 DS board.
Enable them using the common SRIO init code.

Signed-off-by: Li Yang <leoli@freescale.com>
Signed-off-by: Kumar Gala <galak@kernel.crashing.org>