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/st/stm32mp1/stm32mp1.c b/board/st/stm32mp1/stm32mp1.c
index 33cb7f6..0909093 100644
--- a/board/st/stm32mp1/stm32mp1.c
+++ b/board/st/stm32mp1/stm32mp1.c
@@ -19,6 +19,7 @@
 #include <malloc.h>
 #include <misc.h>
 #include <mtd_node.h>
+#include <net.h>
 #include <netdev.h>
 #include <phy.h>
 #include <remoteproc.h>