stmmac: Allow SOCs to use Store forward mode eventhough tx_coe is 0. (V2)

This patch adds new field 'force_sf_dma_mode' to plat_stmmacenet_data
struct to allow users to specify if they want to use force store forward
eventhough tx_coe is not available in hw.
without this flag stmmac driver will use cut-thru mode not use
store-forward mode.

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