target/riscv: Free registers to avoid memory leak

target/riscv: Free registers to avoid memory leak

Tested with SiFive HiFive1 development board.

Change-Id: I96a9a528057fcf9fc54d3da46a672d2cd54c3d5f
Signed-off-by: Marc Schink <openocd-dev@marcschink.de>
Reviewed-on: http://openocd.zylin.com/4885
Tested-by: jenkins
Reviewed-by: Tim Newsome <tim@sifive.com>
Reviewed-by: Tomas Vanek <vanekt@fbl.cz>