net/designware: ensure device private data is DMA aligned.

struct dw_eth_dev contains fields which are accessed via DMA, so make sure it
is aligned to a dma boundary. Without this I see:
    ERROR: v7_dcache_inval_range - start address is not aligned - 0x7fb677e0

Signed-off-by: Ian Campbell <ijc@hellion.org.uk>
Reviewed-by: Alexey Brodkin <abrodkin@synopsys.com>
Acked-by: Marek Vasut <marex@denx.de>