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>