Utilise the new Critical Clock infrastructure to mark clocks which
much not be disabled as CRITICAL.

While we're at it, reduce the coverage of the flex_flags variable,
since it's only really used in a single for() loop.
Signed-off-by: NLee Jones <lee.jones@linaro.org>
Signed-off-by: NStephen Boyd <sboyd@codeaurora.org>