mailbox: pcc: Clear any pending responder interrupts before enabling it

mailbox: pcc: Clear any pending responder interrupts before enabling it

Some platforms may leave a responder interrupt pending from earlier
transactions. If a PCC responder channel has a pending interrupt when
the controller starts up, enabling the IRQ line without first clearing
the condition can lead to a spurious interrupt which could disrupt other
transmissions if the IRQ is shared.

Explicitly clear any pending responder interrupt before enabling the IRQ
to ensure a clean start. Acknowledge the responder channel via
pcc_chan_acknowledge() in startup before requesting/enablement of the
IRQ. This ensures a clean baseline for the first transfer/receiption
of the notification/response.

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>