bt_ctf_clock_class_create(): add a frequency parameter

The frequency of a clock class is a mandatory property: CTF 1.8
indicates that it's 1 GHz by default, but it is expected that there's no
default frequency in CTF 2. To allow this immediately, add a mandatory
frequency parameter to bt_ctf_clock_class_create() so that the user's
frequency is set at creation time.

To maintain backward compatibility with pre-2.0.0 CTF writer,
bt_ctf_clock_create() sets its clock class's frequency to 1 GHz, and so
do other parts of the code base.

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>