kbuild: check libyaml installation for 'make dt_binding_check'

If you run 'make dtbs_check' without installing the libyaml package,
the error message "dtc needs libyaml ..." is shown.

This should be checked also for 'make dt_binding_check' because dtc
needs to validate *.example.dts extracted from *.yaml files.

It is missing since commit 4f0e3a57 ("kbuild: Add support for DT
binding schema checks"), but this fix-up is applicable only after commit
e10c4321 ("kbuild: allow to run dt_binding_check and dtbs_check
in a single command").

I gave the Fixes tag to the latter in case somebody is interested in
back-porting this.

Fixes: e10c4321

 ("kbuild: allow to run dt_binding_check and dtbs_check in a single command")
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Signed-off-by: Rob Herring <robh@kernel.org>