ACPI / CPPC: Support PCC with interrupt flag

For PCC mailbox with interrupt flag, CPPC should call mbox_chan_txdone()
function to notify the mailbox framework about TX completion.

Signed-off-by: Hoan Tran <hotran@apm.com>
Reviewed-by: Prashanth Prakash <pprakash@codeaurora.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>