drivers: ethernet: eth_nxp_enet_qos: reassemble split frames

DMA copies frames which cannot fit into a single buffer into
multiple receive buffers. Updated the receiving code to combine
these buffers into a net_pkt with multiple fragments.

This allows the driver to handle larger Ethernet frames that
span across multiple DMA buffers.

Signed-off-by: Stanislav Poboril <stanislav.poboril@nxp.com>