drivers: eth_mcux allow network buffers to be cached

Do not force network buffers into noncacheable memory, as these buffers
are internal to the driver and not used by the hardware.

Performance change (as tested with iperf on RT1050):
TCP RX: 23.7Mbps->40.6Mbps
TCP TX: 22.3Mbps->36.8Mbps

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