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>

src/jtag/interfaces.c

@@ -113,6 +113,9 @@ extern struct jtag_interface osbdm_interface;
 #if BUILD_OPENDOUS == 1
 extern struct jtag_interface opendous_interface;
 #endif
+#if BUILD_SYSFSGPIO == 1
+extern struct jtag_interface sysfsgpio_interface;
+#endif
 #endif /* standard drivers */
 
 /**
@@ -194,6 +197,9 @@ struct jtag_interface *jtag_interfaces[] = {
 #if BUILD_OPENDOUS == 1
 		&opendous_interface,
 #endif
+#if BUILD_SYSFSGPIO == 1
+		&sysfsgpio_interface,
+#endif
 #endif /* standard drivers */
 		NULL,
 	};