net: rtl8169: Use non-cached memory if available

To work around potential issues with explicit cache maintenance of the
RX and TX descriptor rings, allocate them from a pool of uncached memory
if the architecture supports it.

Signed-off-by: Thierry Reding <treding@nvidia.com>
Signed-off-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Tom Warren <twarren@nvidia.com>