arm: socfpga: mailbox: Refactor mailbox timeout event handling

Add miliseconds delay when waiting for mailbox event to happen
before timeout. This will ensure the timeout duration is predictive.

Signed-off-by: Chee Hong Ang <chee.hong.ang@intel.com>
Reviewed-by: Ley Foon Tan <ley.foon.tan@intel.com>