powerpc/85xx: Rename P2040 id & SERDES to P2041

P2041 is the superset part that covers both P2040 & P2041.  The only
difference between the two devices is that P2041 supports 10g/XAUI and
has an L2 cache.

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