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>

tcl/interface/jtag_vpi.cfg

@@ -0,0 +1,10 @@
+interface jtag_vpi
+
+# Set the VPI JTAG server address
+if { [info exists VPI_PORT] } {
+   set _VPI_PORT $VPI_PORT
+} else {
+   set _VPI_PORT 50020
+}
+
+jtag_vpi_set_port $_VPI_PORT