stmmac: support wake up irq from external sources (v3)

On some platforms e.g. SPEAr the wake up irq differs from the
GMAC interrupt source.
With this patch an external wake up irq can be passed through the
platform code and named as "eth_wake_irq".

In case the wake up interrupt is not passed from the platform
so the driver will continue to use the mac irq (ndev->irq)

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