[prim_lc_sender] Add AsyncOn parameter
[prim_lc_sender] Add AsyncOn parameter

This parameters allows to omit the output flops and replace them by SEC
buffers.

Signed-off-by: Michael Schaffner <a005a2d3342a687ea2d05360c953107acbe83e08@opentitan.org>

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