Fix clock serialization using "frequency" instead of the "freq" field name

Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>
Only use "le" and "be" byte order strings

Babeltrace's parser only handles these two strings when parsing the "trace"
block.

Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>