stmmac: add clk management support

this patch adds the way to enable/disable the MAC
clock when call the open/close and resume/restore
functions.
This has been tested on ST platforms and SPEAr; thanks
to Francesco and Deepak.

Signed-off-by: Deepak Sikri <deepak.sikri@st.com>
Tested-by: Francesco Virlinzi <francesco.virlinzi@st.com>
Signed-off-by: Giuseppe Cavallaro <peppe.cavallaro@st.com>
Signed-off-by: David S. Miller <davem@davemloft.net>