u-boot-tools: drop yaml in DT validation

u-boot-tools: drop yaml in DT validation

Since yamltree was dropped from upstream dtc (i.e., in Linux), a patch was
sent to U-Boot to backport the corresponding changes.

Apply this patch in u-boot's sources.

Overall, this fixes the build-time issues with DTC (and so in
u-boot-tools).

Considering libyaml is no longer used, the dependency on libyaml-native can be
dropped as well (thus reverting the change below):

02e09e036e: u-boot-tools: Add dependency on libyaml for dtschema validation

Signed-off-by: João Marcos Costa <joaomarcos.costa@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>