nios2: System calls handling

This patch adds support for system calls from userspaces. It uses the
asm-generic/unistd.h definitions with architecture spcific syscall.
The sys_call_table is just an array defined in a C file and it contains
pointers to the syscall functions.

Signed-off-by: Ley Foon Tan <lftan@altera.com>

arch/nios2/include/uapi/asm/unistd.h

@@ -0,0 +1,25 @@
+/*
+ * Copyright (C) 2013 Altera Corporation
+ *
+ * This program is free software; you can redistribute it and/or modify it
+ * under the terms and conditions of the GNU General Public License,
+ * version 2, as published by the Free Software Foundation.
+ *
+ * This program is distributed in the hope it will be useful, but WITHOUT
+ * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
+ * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License for
+ * more details.
+ *
+ * You should have received a copy of the GNU General Public License along with
+ * this program.  If not, see <http://www.gnu.org/licenses/>.
+ *
+ */
+
+ #define sys_mmap2 sys_mmap_pgoff
+
+/* Use the standard ABI for syscalls */
+#include <asm-generic/unistd.h>
+
+/* Additional Nios II specific syscalls. */
+#define __NR_cacheflush (__NR_arch_specific_syscall)
+__SYSCALL(__NR_cacheflush, sys_cacheflush)