stmmac: rework normal and enhanced descriptors

Currently the driver assumes that the mac10/100 can only use the
normal descriptor structure and the gmac can only use the
enhanced structures.
This patch removes the descriptor's code from the dma files
and adds two new files just for handling the normal and enhanced
descriptors.

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