stmmac: introduce stmmac_get_platform_resources()

Refactor all code that deals with platform resources into it's
own get function. This function will later be used in the probe
function in dwmac-* drivers.

Signed-off-by: Joachim Eastwood <manabian@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>