dts: yaml: Align serial driver clocks bindings

The common uart.yaml defines an optional clocks property that can be
overridden by specific serial driver bindings to be required. The
description and generation attributes are duplicated.

This fixes build warnings:
extract_dts_includes.py: Merge of 'description': 'Clock gate control
	information'  overwrites 'Clock gate information'.
extract_dts_includes.py: Merge of 'generation': 'structures'  overwrites
	'define'.

And introduces a different build warning, which is intended behavior:
extract_dts_includes.py: Merge of 'category': 'required'  overwrites
	'optional'.

Signed-off-by: Maureen Helm <maureen.helm@nxp.com>