net: phy: marvell: Add a missing errno.h header

This corrects a build error on zynqmp.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/drivers/net/phy/marvell.c b/drivers/net/phy/marvell.c
index 58d287b..4eeb0f6 100644
--- a/drivers/net/phy/marvell.c
+++ b/drivers/net/phy/marvell.c
@@ -8,6 +8,7 @@
  */
 #include <config.h>
 #include <common.h>
+#include <errno.h>
 #include <phy.h>
 
 #define PHY_AUTONEGOTIATE_TIMEOUT 5000