Currently, stmmac only supports 32 bits addressing for SKB. Enable the
support for upto 48 bits addressing in XGMAC core.

This avoids the use of bounce buffers and increases performance.

Changes from v1:
	- Fallback to 32 bits in failure (Andrew)
Signed-off-by: Jose Abreu <joabreu@synopsys.com>
Cc: Joao Pinto <jpinto@synopsys.com>
Cc: David S. Miller <davem@davemloft.net>
Cc: Giuseppe Cavallaro <peppe.cavallaro@st.com>
Cc: Alexandre Torgue <alexandre.torgue@st.com>
Cc: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>