dt-bindings: pmem: Convert binding to YAML

dt-bindings: pmem: Convert binding to YAML

Convert the PMEM device tree binding from text to YAML. This will allow
device trees with pmem-region nodes to pass dtbs_check.

[iweiny: fix ups from Rob/Drew]

Acked-by: Conor Dooley <conor.dooley@microchip.com>
Acked-by: Oliver O'Halloran <oohall@gmail.com>
Signed-off-by: Drew Fustini <drew@pdp7.com>
Acked-by: Dan Williams <dan.j.williams@intel.com>
Link: https://patch.msgid.link/20250606184405.359812-4-drew@pdp7.com
Signed-off-by: Ira Weiny <ira.weiny@intel.com>