This adds some missing CSR_* register macros, and documents some as being
priv v1.9.1 specific.

Signed-off-by: Jim Wilson <jimw@sifive.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Message-Id: <20190212230830.9160-1-jimw@sifive.com>
Signed-off-by: Palmer Dabbelt <palmer@sifive.com>