Add RISC-V support.

This supports both 0.11 and 0.13 versions of the debug spec.

Support for `-rtos riscv` will come in a separate commit since it was
easy to separate out, and is likely to be more controversial.

Flash support for the SiFive boards will also come in a later commit.

Change-Id: I1d38fe669c2041b4e21a5c54a091594aac3e2190
Signed-off-by: Tim Newsome <tim@sifive.com>
Reviewed-on: http://openocd.zylin.com/4578
Tested-by: jenkins
Reviewed-by: Liviu Ionescu <ilg@livius.net>
Reviewed-by: Matthias Welwarsky <matthias@welwarsky.de>

tcl/board/sifive-e31arty.cfg

@@ -0,0 +1,22 @@
+#
+# Be sure you include the speed and interface before this file
+# Example:
+# -c "adapter_khz 5000" -f "interface/ftdi/olimex-arm-usb-tiny-h.cfg" -f "board/sifive-e31arty.cfg"
+
+set _CHIPNAME riscv
+jtag newtap $_CHIPNAME cpu -irlen 5 -expected-id 0x20000001
+
+set _TARGETNAME $_CHIPNAME.cpu
+
+target create $_TARGETNAME.0 riscv -chain-position $_TARGETNAME
+$_TARGETNAME.0 configure -work-area-phys 0x80000000 -work-area-size 10000 -work-area-backup 1
+
+flash bank spi0 fespi 0x40000000 0 0 0 $_TARGETNAME.0 0x20004000
+init
+if {[ info exists pulse_srst]} {
+  ftdi_set_signal nSRST 0
+  ftdi_set_signal nSRST z
+}
+halt
+flash protect 0 64 last off
+echo "Ready for Remote Connections"