mv643xx_eth: Add Rx Discard and Rx Overrun statistics

These statistics helped me a lot while searching who is losing
packets in my setup.
I added these stats to MIB group since they are very similar,
but just in other registers.
I have tested this patch on 88F6281 SoC.

Signed-off-by: Paulius Zaleckas <paulius.zaleckas@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>