[POWERPC] 83xx: Updated and renamed MPC8360PB to MPC836x MDS

The MPC836x PB board is really just one part of the MPC836x MDS.  We currently
name all other PB boards as MDS.  Removed all references to PB and replaced
with MDS.  Additionally renamed the .dts to match the defconfig (mpc836x_mds*).

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