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/bcmgenet.c b/drivers/net/bcmgenet.c
index e971b55..b7b0f57 100644
--- a/drivers/net/bcmgenet.c
+++ b/drivers/net/bcmgenet.c
@@ -16,6 +16,7 @@
  * we only support v5, as used in the Raspberry Pi 4.
  */
 
+#include <asm/cache.h>
 #include <asm/io.h>
 #include <clk.h>
 #include <cpu_func.h>