mailbox: pcc: Avoid accessing PCCT table in pcc_send_data and pcc_mbox_irq

Now that the con_priv is availvale solely for PCC mailbox controller
driver, let us use the same to save the channel specific information
in it so that we can it whenever required instead of parsing the PCCT
table entries every time in both pcc_send_data and pcc_mbox_irq.

We can now use the newly introduces PCC register bundle to simplify both
saving of channel specific information and accessing them without repeated
checks for the subspace type.

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>