mailbox: bcm-pdc: Fix some kernel-doc comments

Fix some kernel-doc comments to silence the warnings:

drivers/mailbox/bcm-pdc-mailbox.c:707: warning: Function parameter or member 'pdcs' not described in 'pdc_tx_list_sg_add'
drivers/mailbox/bcm-pdc-mailbox.c:707: warning: Excess function parameter 'spu_idx' description in 'pdc_tx_list_sg_add'
drivers/mailbox/bcm-pdc-mailbox.c:875: warning: Function parameter or member 'pdcs' not described in 'pdc_rx_list_sg_add'
drivers/mailbox/bcm-pdc-mailbox.c:875: warning: Excess function parameter 'spu_idx' description in 'pdc_rx_list_sg_add'
drivers/mailbox/bcm-pdc-mailbox.c:966: warning: Function parameter or member 't' not described in 'pdc_tasklet_cb'
drivers/mailbox/bcm-pdc-mailbox.c:966: warning: Excess function parameter 'data' description in 'pdc_tasklet_cb'

Signed-off-by: Yang Li <yang.lee@linux.alibaba.com>
Signed-off-by: Jassi Brar <jaswinder.singh@linaro.org>