net: stmmac:sti: Add STi SOC glue driver.

STi series SOCs have a glue layer on top of the synopsis gmac IP, this
glue layer needs to be configured before the gmac driver starts using
the IP.

This patch adds a support to this glue layer which is configured via
stmmac setup, init, exit callbacks.

Signed-off-by: Srinivas Kandagatla <srinivas.kandagatla@st.com>
Signed-off-by: David S. Miller <davem@davemloft.net>