dt-bindings: net: dwmac: fix queue priority documentation

The priority field is not the queue priority (queue priority is fixed)
but a bitmask of priorities assigned to this queue.

In receive, priorities relate to tagged frames priorities.

In transmit, priorities relate to PFC frames.

Signed-off-by: Seb Laveze <sebastien.laveze@nxp.com>
Link: https://lore.kernel.org/r/20210111081406.1348622-1-sebastien.laveze@oss.nxp.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>