powerpc/85xx: Add is_serdes_configured() support for MPC8572 SERDES

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.

Signed-off-by: Chenhui Zhao <b26998@freescale.com>
Signed-off-by: Kumar Gala <galak@kernel.crashing.org>