dts: bindings: Clean up microchip,xec-espi.yaml and espi.yaml

 - Remove 'generation:'

 - Remove 'version:' (see
   https://github.com/zephyrproject-rtos/zephyr/pull/17681)

 - Remove redundant document separators (see
   https://github.com/zephyrproject-rtos/zephyr/pull/16913

)

 - Use base.yaml to define common properties and property settings

Signed-off-by: Ulf Magnusson <Ulf.Magnusson@nordicsemi.no>