[ARM] 4714/2: Headers for IXP4xx built-in Ethernet and WAN drivers

Adds platform structs and #defines required by drivers for
IXP4xx built-in Ethernet and WAN (sync serial) ports.

The actual drivers will reside in drivers/net/arm and
drivers/net/wan and will be submitted separately.

Signed-off-by: Krzysztof Halasa <khc@pm.waw.pl>
Signed-off-by: Rod Whitby <rod@whitby.id.au>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>