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 <[email protected]>
Reviewed-by: Andrew Jones <[email protected]>
Signed-off-by: Anup Patel <[email protected]>

Author: xiaobo55x