drivers: eth: eth_sam_gmac: Fix priority queues

The Atmel SAM SoC with ethernet port uses same GMAC driver. However,
there are differences between SoC GMAC implementation. Some SoCs have
priority queue and system can configure 0 up to 5, depending of SoC
version. This update current GMAC driver adding missing definitions.

Co-authored-by: Stephanos Ioannidis <root@stephanos.io>
Signed-off-by: Gerson Fernando Budke <nandojve@gmail.com>