drivers: new jtag bitbang driver using sysfs gpio

This driver implements a bitbang jtag interface using gpio lines exported via
sysfs.

The aim of this driver implementation is to use system GPIOs but to avoid the
need for an additional kernel driver.

A config suitable for RaspberryPi is included.

Change-Id: Ib2acf720247a219768d1cbfeebd88057ed2d7b8b
Signed-off-by: Marc Reilly <marc@cpdesign.com.au>
Reviewed-on: http://openocd.zylin.com/762
Reviewed-by: Spencer Oliver <spen@spen-soft.co.uk>
Tested-by: jenkins
Reviewed-by: Freddie Chopin <freddie.chopin@gmail.com>

tcl/interface/sysfsgpio-raspberrypi.cfg

@@ -0,0 +1,24 @@
+#
+# Config for using RaspberryPi's expansion header
+#
+# This is best used with a fast enough buffer but also
+# is suitable for direct connection if the target voltage
+# matches RPi's 3.3V
+#
+# Do not forget the GND connection, pin 6 of the expansion header.
+#
+
+interface sysfsgpio
+
+# This has no effect on the driver, but is required
+adapter_khz 100
+
+# Each of the JTAG lines need a gpio number set: tck tms tdi tdo
+# Header pin numbers: 23 22 19 21
+sysfsgpio_jtag_nums 11 25 10 9
+
+# At least one of srst or trst needs to be specified
+# Header pin numbers: TRST - 26, SRST - 18
+sysfsgpio_trst_num 7
+# sysfsgpio_srst_num 24
+