[multitop] Add clock measurement extension field to the clkmgr DT

The `clkmgr` DT extension current does not contain any information about the clock measurement registers. This the clock list is top-specific, this should be added so that the relevant tests can be ported to multitop without hacks.

Start by locating the clkmgr device-tree extension and the relevant clock-measurement tests; compare the existing clock-list data with what multitop needs. Done means the extension exposes clock measurement register information and the relevant tests can be ported to multitop without hacks.

Written by the indexing model from the issue text.