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>

diff --cc arch/nios2/include/asm/syscall.h
Simple merge

diff --cc arch/nios2/include/asm/syscalls.h
Simple merge

diff --cc arch/nios2/include/uapi/asm/unistd.h
Simple merge

diff --cc arch/nios2/kernel/sys_nios2.c
Simple merge

diff --cc arch/nios2/kernel/syscall_table.c
Simple merge