[POWERPC] 83xx: Updated and renamed MPC834x SYS to MPC834x MDS

The MPC834x SYS board has always been called the MPC834x MDS since its public
release.  Removed all references to SYS and replaced with MDS.  Additionally
renamed the .dts to match the defconfig (mpc834x_mds*).

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