stmmac: share reset function between dwmac100 and dwmac1000

This patch is to share the same reset procedure between dwmac100 and
dwmac1000 chips.
This will also help on enhancing the driver and support new chips.

Signed-off-by: Giuseppe Cavallaro <peppe.cavallaro@st.com>
Signed-off-by: Alexandre TORGUE <alexandre.torgue@st.com>
Signed-off-by: David S. Miller <davem@davemloft.net>