davinci_emac: fix for running with dcache enabled

davinci_emac: fix for running with dcache enabled

DaVinci EMAC is present on TI AM35xx SoCs (ARMv7) which run with D-Cache
enabled by default. So we have to take care and flush/invalidate the
cache before/after the DMA operations.

Please note that the receive buffer alignment to 32 byte boundary comes
from the old driver version I don't know if it is really needed or
alignment to cache line size is enough.

Signed-off-by: Ilya Yanok <yanok@emcraft.com>