pcnet: align rx buffers for cache invalidation

The RX buffers are invalidated when a packet is received, however they
were not suitably cache-line aligned. Allocate them seperately to the
pcnet_priv structure and align to ARCH_DMA_MINALIGN in order to ensure
suitable alignment for the cache invalidation, preventing anything else
being placed in the same lines & lost.

Signed-off-by: Paul Burton <paul.burton@imgtec.com>