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/synopsys/hsdk/hsdk.c b/board/synopsys/hsdk/hsdk.c
index 8ccd84c..e32e782 100644
--- a/board/synopsys/hsdk/hsdk.c
+++ b/board/synopsys/hsdk/hsdk.c
@@ -10,6 +10,7 @@
 #include <env.h>
 #include <init.h>
 #include <irq_func.h>
+#include <asm/cache.h>
 #include <linux/printk.h>
 #include <linux/kernel.h>
 #include <linux/io.h>