net/stmmac: Move "#include <linux/platform_device.h>" to linux/stmmac.h

stmmac.h uses struct platform_device and doesn't include
<linux/platform_device.h>. Whereas drivers/net/stmmac/stmmac.h includes it, but
doesn't directly use it. And so we get following compilation warning while using
this file:
	warning: ‘struct platform_device’ declared inside parameter list

This patch includes <linux/platform_device.h> in linux/stmmac.h and removes it
from drivers/net/stmmac/stmmac.h

Signed-off-by: Viresh Kumar <viresh.kumar@st.com>
Acked-by: Giuseppe Cavallaro <peppe.cavallaro@st.com>
Signed-off-by: David S. Miller <davem@davemloft.net>