[POWERPC] mpc512x: Device tree for MPC5121 ADS

Minimal /dts-v1/ device tree for mpc5121 ads.

port-number property in uart nodes
will go away after the driver learns to use aliases

Signed-off-by: John Rigby <jrigby@freescale.com>
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>