powerpc/85xx: Rework MPC8536 SERDES is_serdes_configured support

Move serdes init until after we are in ram so we can keep track of a
global static protocal map for the particular serdes config we are in.
This makes is_serdes_configured() much simplier and not constantly
reading registers to determine if a given device is enabled based on the
protocol.

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