tools: riscv: Add header file csr.h

Borrow the csr definitions and operations from kernel's
arch/riscv/include/asm/csr.h to tools/ for riscv.
Signed-off-by: Haibo Xu <haibo1.xu@intel.com>
Reviewed-by: Andrew Jones <ajones@ventanamicro.com>
Signed-off-by: Anup Patel <anup@brainfault.org>