[CDC] Define AST_BYPASS_CLK
[CDC] Define AST_BYPASS_CLK

Issue #11492

ast SYNTHESIS does not generate any meaningful clock as they are
replaced to the .db model. Due to the clock, CDC run-time is increased
significantly.

This commit rolls back the AST_BYPASS_CLK definition for a temporary
fix.

The proper solution will be discussed in the issue #11492.

Signed-off-by: Eunchan Kim <cb7bc7ceb22698f298c67faa994fd9ddf3cab298@opentitan.org>

lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan