net: dwc_eth_qos: Pad descriptors to cacheline size

The DWMAC4 IP has the possibility to skip up to 7 AXI bus width size words
after the descriptor. Use this to pad the descriptors to cacheline size and
remove the need for noncached memory altogether. Moreover, this lets Tegra
use the generic cache flush / invalidate operations.

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Joe Hershberger <joe.hershberger@ni.com>
Cc: Patrice Chotard <patrice.chotard@st.com>
Cc: Patrick Delaunay <patrick.delaunay@st.com>
Cc: Ramon Fried <rfried.dev@gmail.com>
Cc: Stephen Warren <swarren@nvidia.com>
Tested-by: Stephen Warren <swarren@nvidia.com>
Reviewed-by: Stephen Warren <swarren@nvidia.com>
Tested-by: Patrice Chotard <patrice.chotard@foss.st.com>