Clarify what exactly the RISC-V code supports.

Clarify what exactly the RISC-V code supports.

Change-Id: I8da657426cc52c738ab41bfb0164cbc6721c0aef
Signed-off-by: Tim Newsome <tim@sifive.com>
Reviewed-on: http://openocd.zylin.com/4655
Tested-by: jenkins
Reviewed-by: Philipp Guehring <pg@futureware.at>
Reviewed-by: Liviu Ionescu <ilg@livius.net>
Reviewed-by: Tomas Vanek <vanekt@fbl.cz>