mailbox: pcc: Set txdone_irq/txdone_poll based on PCCT flags

mailbox: pcc: Set txdone_irq/txdone_poll based on PCCT flags

The PCC controller currently enables txdone via IRQ if the PCCT exposes
platform capability to generate command completion interrupt, but it
leaves txdone_poll unchanged. Make the behaviour explicit:

  - If ACPI_PCCT_DOORBELL is present, use txdone_irq and disable polling.
  - Otherwise, disable txdone_irq and fall back to txdone_poll.

Configure the PCC mailbox to use interrupt-based completion for PCC types
that signal completion via IRQ using TXDONE_BY_IRQ, and fall back to
polling for others using TXDONE_BY_POLL.

This ensures the PCC driver uses the appropriate completion mechanism
according to the PCCT table definition and makes the completion mode
unambiguous avoiding mixed signalling when the platform lacks a doorbell
flag set.

Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>
Tested-by: Adam Young <admiyo@os.amperecomputing.com>
Signed-off-by: Jassi Brar <jassisinghbrar@gmail.com>