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/drivers/net/pcnet.c b/drivers/net/pcnet.c
index f97e7f8..b536ad4 100644
--- a/drivers/net/pcnet.c
+++ b/drivers/net/pcnet.c
@@ -11,6 +11,7 @@
 #include <malloc.h>
 #include <net.h>
 #include <netdev.h>
+#include <asm/cache.h>
 #include <asm/io.h>
 #include <pci.h>