powerpc: dts: mpc83xx: Add unit addresses to /memory

This fixes dtschema warnings such as the following:

  arch/powerpc/boot/dts/mpc8315erdb.dtb: /: memory: False schema
  does not allow {'device_type': ['memory'], 'reg': [[0, 134217728]]}

Signed-off-by: J. Neuschäfer <j.ne@posteo.net>
Signed-off-by: Madhavan Srinivasan <maddy@linux.ibm.com>
Link: https://patch.msgid.link/20260303-mpc83xx-cleanup-v2-5-187d3a13effa@posteo.net