mailbox/pcc: support mailbox management of the shared buffer

Define a new, optional, callback that allows the driver to
specify how the return data buffer is allocated.  If that callback
is set,  mailbox/pcc.c is now responsible for reading from and
writing to the PCC shared buffer.

This also allows for proper checks of the Commnand complete flag
between the PCC sender and receiver.

For Type 4 channels, initialize the command complete flag prior
to accepting messages.

Since the mailbox does not know what memory allocation scheme
to use for response messages, the client now has an optional
callback that allows it to allocate the buffer for a response
message.

When an outbound message is written to the buffer, the mailbox
checks for the flag indicating the client wants an tx complete
notification via IRQ.  Upon receipt of the interrupt It will
pair it with the outgoing message. The expected use is to
free the kernel memory buffer for the previous outgoing message.

Signed-off-by: Adam Young <admiyo@os.amperecomputing.com>
Signed-off-by: Jassi Brar <jassisinghbrar@gmail.com>