kbuild: fix a typo dt_binding_schema -> dt_binding_schemas

kbuild: fix a typo dt_binding_schema -> dt_binding_schemas

If we follow "make help" to "make dt_binding_schema", we will see
below error:

$ make dt_binding_schema
make[1]: *** No rule to make target 'dt_binding_schema'.  Stop.
make: *** [Makefile:224: __sub-make] Error 2

It should be a typo. So this will fix it.

Fixes: 604a57ba9781 ("dt-bindings: kbuild: Add separate target/dependency for processed-schema.json")
Signed-off-by: Xu Yang <xu.yang_2@nxp.com>
Reviewed-by: Nicolas Schier <n.schier@avm.de>
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>