[pwrmgr] rdc fixes
[pwrmgr] rdc fixes

- Handle the case where a power glitch can cause the main power state
  to change asynchronously.

Signed-off-by: Timothy Chen <d7b2ae30d8c0f215facd721941273067fd1d14a2@google.com>

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