drivers: eth_mcux: move dma buffers and buffer descriptors to DTCM

Move dma buffers and buffer descriptors to DTCM, to improve hardware
and software access speed during network TX/RX

Performance change (as tested with iperf on RT1050)
TCP RX: 40.6Mbps->44.6Mbps
TCP TX: 36.8Mbps->38.1Mbps

Signed-off-by: Daniel DeGrasse <daniel.degrasse@nxp.com>