riscv: ignore nanosleep syscall

Change-Id: I564a09564da668a5db3e75f15b33efaca363d71a
Reviewed-on: https://gem5-review.googlesource.com/c/9624
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>
Maintainer: Jason Lowe-Power <jason@lowepower.com>