FEC: Allow multiple FECes

This patch allows user to register multiple FEC controllers. To preserve
compatibility with older boards, the mxcfec_register() call is still in place.
To use multiple controllers, new macro is in place, the mxcfec_register_multi(),
which takes more arguments. The syntax is:

mxcfec_register_multi(bd, FEC ID, FEC PHY ID on the MII bus, base address);

To disable the fecmxc_register() compatibility stuff, define the macro
CONFIG_FEC_MXC_MULTI. This will remove the requirement for defining IMX_FEC_BASE
and CONFIG_FEC_MXC_PHYADDR.

Signed-off-by: Marek Vasut <marek.vasut@gmail.com>
Cc: Ben Warren <biggerbadderben@gmail.com>
Cc: Stefano Babic <sbabic@denx.de>
Cc: Wolfgang Denk <wd@denx.de>
Cc: Detlev Zundel <dzu@denx.de>