net: ethernet: stmmac: Add support for syscfg clock

Add optional support for syscfg clock in dwmac-stm32.c
Now Syscfg clock is activated automatically when syscfg
registers are used

Signed-off-by: Christophe Roullier <christophe.roullier@st.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>