mailbox: pcc: Use PCC mailbox channel pointer instead of standard

Now that we have all the shared memory region information populated in
the pcc_mbox_chan, let us propagate the pointer to the same as the
return value to pcc_mbox_request channel.

This eliminates the need for the individual users of PCC mailbox to
parse the PCCT subspace entries and fetch the shmem information. This
also eliminates the need for PCC mailbox controller to set con_priv to
PCCT subspace entries. This is required as con_priv is private to the
controller driver to attach private data associated with the channel and
not meant to be used by the mailbox client/users.

Let us convert all the users of pcc_mbox_{request,free}_channel to use
new interface.

Cc: Jean Delvare <jdelvare@suse.com>
Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>
Acked-by: Wolfram Sang <wsa@kernel.org>
Acked-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Jassi Brar <jaswinder.singh@linaro.org>