Setups with digital audio connections like SPDIF and ADAT require
a designated master clock to stay in sync. Add a sysctl setting
to control the preferred clock source for each HDSPe sound card.
Complement this by sysctl values to list available clock sources,
show the currently effective clock source and display the sync
status of all connections. Clock sources are named according to
RME user manuals.

Submitted by: Florian Walpen <dev@submerge.ch>
Differential Revision: https://reviews.freebsd.org/D43252