edtlib: fix last modified semantic in included property specs #1149

Job	Run time
Devicetree script tests (3.8, ubuntu-22.04)	27s
Devicetree script tests (3.8, macos-11)	59s
Devicetree script tests (3.8, windows-2022)	1m 13s
Devicetree script tests (3.9, ubuntu-22.04)	28s
Devicetree script tests (3.9, macos-11)	1m 0s
Devicetree script tests (3.9, windows-2022)	1m 5s
Devicetree script tests (3.10, ubuntu-22.04)	28s
Devicetree script tests (3.10, macos-11)	1m 3s
Devicetree script tests (3.10, windows-2022)	56s
Devicetree script tests (3.11, ubuntu-22.04)	33s
Devicetree script tests (3.11, macos-11)	1m 3s
Devicetree script tests (3.11, windows-2022)	53s
Devicetree script tests (3.12, ubuntu-22.04)	30s
Devicetree script tests (3.12, macos-11)	1m 14s
Devicetree script tests (3.12, windows-2022)	1m 19s
	13m 11s

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