[DRIVERS] Add dummy <linux/unistd.h> for driver porting

This patch adds dummy (or place holder) linux/unistd.h for fixing
compilation drivers/net/phy/phy.c

Signed-off-by: Anup Patel <anup@brainfault.org>

drivers/include/linux/unistd.h

@@ -0,0 +1,8 @@
+#ifndef _LINUX_UNISTD_H
+#define _LINUX_UNISTD_H
+
+#include <linux/types.h>
+
+/* FIXME: This file is just a place holder for most cases */
+
+#endif