Export a sysctl dev.<clkdom>.<unit>.clocks for each clock domain containing
all the clocks that they provide.
Each clocks are exported under the node 'clock.<clkname>' and have the following
children nodes :

• frequency
• parent (The selected parent, if any)
• parents (The list of parents, if any)
• childrens (The list of childrens, if any)
• enable_cnt (The enabled counter)

This give us the possibility to examine clocks at runtime and make graph of
the clock flow.

Reviewed by: mmel
MFC after: 2 month
Differential Revision: https://reviews.freebsd.org/D9833