scripts/dtc: drop yaml in DT validation

scripts/dtc: drop yaml in DT validation

The build issues found in dtc/yamltree were partially mitigated by a
previous commit (807bcd844a: "scripts/dtc: Fix pkg-config behavior under
sysroot"), but upstream dtc simply disabled yaml, and the same should be
done here in order to permanently avoid those issues.

Backport the change below from Linux v5.18 [1]:

ef8795f3f1c ("dt-bindings: kbuild: Use DTB files for validation")

I tested this patch with a couple Yocto builds: u-boot and u-boot-tools,
using the current master branch (rev. "c53b0708f9"), having removed
libyaml-native from u-boot-tools's dependencies.

[1] https://git.kernel.org/linus/ef8795f3f1ce

Signed-off-by: João Marcos Costa <joaomarcos.costa@bootlin.com>
Acked-by: Quentin Schulz <quentin.schulz@cherry.de>