stmmac: add HW DMA feature register (v3)

New GMAC chips have an extra register to indicate
the presence of the optional features/functions of
the DMA core.

This patch adds this support and all the HW cap
are exported via debugfs.

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