stmmac: get the descriptor structure from platform

Output for chip that uses the Enhanced descriptors:
[snip]
STMMAC driver:
	platform registration... done!
	DWMAC1000 - user ID: 0x10, Synopsys ID: 0x33
	Enhanced descriptor structure
	no valid MAC address;please, use ifconfig or nwhwconfig!
	eth0 - (dev. name: stmmaceth - id: 0, IRQ #134
	IO base addr: 0xfd110000)
STMMAC MII Bus: probed
[snip]

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