net: dcb: add helper functions to retrieve PCP and DSCP rewrite maps

Add two new helper functions to retrieve a mapping of priority to PCP
and DSCP bitmasks, where each bitmap contains ones in positions that
match a rewrite entry.

dcb_ieee_getrewr_prio_dscp_mask_map() reuses the dcb_ieee_app_prio_map,
as this struct is already used for a similar mapping in the app table.

Signed-off-by: Daniel Machon <daniel.machon@microchip.com>
Reviewed-by: Petr Machata <petrm@nvidia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>