clk: st: clkgen-pll: Detect critical clocks

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

Clocks are marked as CRITICAL using clk flags.  This patch also
ensures flags are peculated through the framework in the correct
manner.

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