stmmac: rename the gmac as dwmac1000 and split core and dma parts

Use dwmac1000 naming instead of gmac.
The patch also splits the gmac.c file in two new ones:
dwmac1000_core.c and dwmac1000_dma.c.
This could actually help on some architectures where different
DMA engines are used.

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