dts: bindings: improve base.yaml descriptions

This file defines some of the most important properties in the
devicetree, including core concepts like 'compatible', 'reg', and
'interrupts'. For some reason, we've never prioritized writing up
proper descriptions that indicate their importance or show where the
user can get more information about them in the specs or elsewhere.

Give this file a once-over to improve the situation.

Signed-off-by: Martí Bolívar <mbolivar@amperecomputing.com>
Signed-off-by: Martí Bolívar <marti.bolivar@oss.qualcomm.com>