powerpc/86xx: Add SERDES support on MPC8641 & MPC8610

Add the ability to determine if a given IP block connected on SERDES is
configured.  This is useful for things like PCIe and SRIO since they are
only ever connected on SERDES.  This mimics the code we have in place
for the 85xx platforms.

Signed-off-by: Kumar Gala <galak@kernel.crashing.org>