build(deps): bump dtschema from 2026.4 to 2026.6

Bumps [dtschema](https://github.com/devicetree-org/dt-schema) from 2026.4 to 2026.6.
- [Release notes](https://github.com/devicetree-org/dt-schema/releases)
- [Commits](devicetree-org/dt-schema@v2026.04...v2026.06)

---
updated-dependencies:
- dependency-name: dtschema
  dependency-version: '2026.6'
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

Signed-off-by: dependabot[bot] <support@github.com>

Author: dependabot[bot]

requirements.txt

@@ -17,5 +17,5 @@ oras == 0.2.42         # for OCI stuff in mapper-oci-update
 Jinja2 == 3.1.6        # for templating
 rich == 15.0.0         # for rich text formatting
 Pygments == 2.20.0     # syntax highlighting, required by rich.syntax (imported by lib/tools/patching.py)
-dtschema == 2026.4     # for checking dts files and dt bindings
+dtschema == 2026.6     # for checking dts files and dt bindings
 yamllint == 1.38.0     # for checking dts files and dt bindings