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/arch/nds32/lib/cache.c b/arch/nds32/lib/cache.c
index e11d300..21917e5 100644
--- a/arch/nds32/lib/cache.c
+++ b/arch/nds32/lib/cache.c
@@ -8,6 +8,7 @@
 #include <common.h>
 #include <cpu_func.h>
 #if !(CONFIG_IS_ENABLED(SYS_ICACHE_OFF) && CONFIG_IS_ENABLED(SYS_DCACHE_OFF))
+#include <asm/cache.h>
 static inline unsigned long CACHE_SET(unsigned char cache)
 {
 	if (cache == ICACHE)