clk: st: clk-flexgen: Detect critical clocks

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: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>