dt:net:stmmac: Allocate platform data only if its NULL.

In some DT use-cases platform data might be already allocated and passed
via AUXDATA. These are the cases where machine level code populates few
callbacks in the platform data.

This patch adds check and reuses platform_data if its valid, before
allocating a new one.

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