common: Move reset_phy() to net.h

This is a network function so let's move it into that header.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/board/zyxel/nsa310s/nsa310s.c b/board/zyxel/nsa310s/nsa310s.c
index e7cb6be..82a3fee 100644
--- a/board/zyxel/nsa310s/nsa310s.c
+++ b/board/zyxel/nsa310s/nsa310s.c
@@ -7,6 +7,7 @@
 
 #include <common.h>
 #include <miiphy.h>
+#include <net.h>
 #include <asm/arch/cpu.h>
 #include <asm/arch/soc.h>
 #include <asm/arch/mpp.h>