mailbox: pcc: Add pcc_mbox_chan structure to hold shared memory region info

Currently PCC mailbox controller sets con_priv in each channel to hold
the pointer to pcct subspace entry it corresponds to. The mailbox user
will then fetch this pointer from the channel descriptor they get when
they request for the channel. Using that pointer they then parse the
pcct entry again to fetch all the information about shared memory region.

In order to remove individual users of PCC mailbox parsing the PCCT
subspace entries to fetch same information, let us consolidate the same
in pcc mailbox controller by parsing all the shared memory region
information into a structure that can also hold the mbox_chan pointer it
represent.

This can then be used as main PCC mailbox channel pointer that we can
return as part of pcc_mbox_request_channel instead of standard mailbox
channel pointer.

Reviewed-by: Cristian Marussi <cristian.marussi@arm.com>
Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>
Signed-off-by: Jassi Brar <jaswinder.singh@linaro.org>