scripts/dts/extract: Don't warn for 'category' merge of 'required'

We get warnings like:
	foo.yaml('BAH') merge of property 'category': 'required'
	overwrites 'optional'

This warning isn't meaningful as its reasonable to change the 'category'
of a property from 'optional' to 'required'.  So don't warn in the case
anymore.

Signed-off-by: Kumar Gala <kumar.gala@linaro.org>