cpu/or1k: Document no branch delay slot architectures and l.adrp

The 'nd' architectures did not mention what the 'nd' stands for.
Document that these mean 'no brach delay slot'.

cpu/ChangeLog:

yyyy-mm-dd  Stafford Horne  <shorne@gmail.com>

	* or1k.cpu (or64nd, or32nd, or1200nd): Update comment.a
	(l-adrp): Improve comment.