mailbox: bcm-pdc: remove unused struct 'pdc_dma_map'

'pdf_dma_map' has been unused since the original
commit a24532f8 ("mailbox: Add Broadcom PDC mailbox driver").

Remove it.

Signed-off-by: Dr. David Alan Gilbert <linux@treblig.org>
Signed-off-by: Jassi Brar <jassisinghbrar@gmail.com>