jtag: drivers: Add JTAP VPI client driver

jtag: drivers: Add JTAP VPI client driver

This patch adds a driver for the jtag_vpi server [1]. This server is
now part of the ORPSoC version 3 (OpenRISC Reference Platform SoC).
The jtag_vpi server provides an interface between OpenOCD and a simulated
core.

[1] http://github.com/fjullien/jtag_vpi

Change-Id: I717b72cace4845f66c878581345074f99002e21a
Signed-off-by: Franck Jullien <franck.jullien@gmail.com>
Reviewed-on: http://openocd.zylin.com/1609
Tested-by: jenkins
Reviewed-by: Spencer Oliver <spen@spen-soft.co.uk>