mips: Fix compiler warning in cpu.c

mips: Fix compiler warning in cpu.c

There really is zero reason for including netdev.h in generic mips CPU code.
Removing the netdev.h from cpu.c also fixes the following compiler warning:

In file included from arch/mips/cpu/cpu.c:10:0:
include/netdev.h:204:41: warning: 'struct eth_device' declared inside parameter list [enabled by default]
 int fecmxc_register_mii_postcall(struct eth_device *dev, int (*cb)(int));
                                         ^
include/netdev.h:204:41: warning: its scope is only this definition or declaration, which is probably not what you want [enabled by default]

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Paul Burton <paul.burton@imgtec.com>
Cc: Daniel Schwierzeck <daniel.schwierzeck@gmail.com>