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/board/ti/j721e/evm.c b/board/ti/j721e/evm.c
index c068bb8..7bc03ed 100644
--- a/board/ti/j721e/evm.c
+++ b/board/ti/j721e/evm.c
@@ -9,6 +9,7 @@
 
 #include <common.h>
 #include <init.h>
+#include <net.h>
 #include <asm/arch/sys_proto.h>
 #include <asm/arch/hardware.h>
 #include <asm/gpio.h>