clk: Improve flags doc for of_clk_detect_critical()

The "flags" parameter passed to of_clk_detect_critical() cannot be a
pointer to a real clk_core.flags field, as clk_core is private to the
clock framework internals.

Change the comment to refer to top-level framework flags instead.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>