mailbox: pcc: Fix kernel doc warnings

Kernel doc validation script is unhappy and complains with the below set
of warnings.

  |  drivers/mailbox/pcc.c:179: warning: Function parameter or member 'irq'
  |	not described in 'pcc_mbox_irq'
  |  drivers/mailbox/pcc.c:179: warning: Function parameter or member 'p'
  |	not described in 'pcc_mbox_irq'
  |  drivers/mailbox/pcc.c:378: warning: expecting prototype for
  |	parse_pcc_subspaces(). Prototype was for parse_pcc_subspace() instead

Fix it.

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