stmmac: split to core library and probe drivers

Instead of registering the platform and PCI drivers in one module let's move
necessary bits to where it belongs. During this procedure we convert the module
registration part to use module_*_driver() macros which makes code simplier.

>From now on the driver consists three parts: core library, PCI, and platform
drivers.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Acked-by: Giuseppe Cavallaro <peppe.cavallaro@st.com>
Signed-off-by: David S. Miller <davem@davemloft.net>