common: Drop net.h from common header

Move this header out of the common header. Network support is used in
quite a few places but it still does not warrant blanket inclusion.

Note that this net.h header itself has quite a lot in it. It could be
split into the driver-mode support, functions, structures, checksumming,
etc.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/arch/m68k/cpu/mcf523x/cpu.c b/arch/m68k/cpu/mcf523x/cpu.c
index 4297819..8c808a6 100644
--- a/arch/m68k/cpu/mcf523x/cpu.c
+++ b/arch/m68k/cpu/mcf523x/cpu.c
@@ -9,6 +9,7 @@
  */
 
 #include <common.h>
+#include <net.h>
 #include <vsprintf.h>
 #include <watchdog.h>
 #include <command.h>