powerpc/microwatt: Fix model property in device tree

The standard property for the model name is called "model".

Signed-off-by: J. Neuschäfer <j.ne@posteo.net>
Signed-off-by: Madhavan Srinivasan <maddy@linux.ibm.com>
Link: https://patch.msgid.link/20250611-microwatt-v2-1-80847bbc5f9c@posteo.net