drivers/jtag: usb_blaster add pin controls

Add pin controls (pin6 and pin8). These pins can be
controled either :
 - with a fixed value (0 or 1)
 - by TRST or SRST reset signal (driven low when signal
 activated).

Change-Id: Ie6a3e57fc6ba6a38fdb9a009c976e3febab8f578
Signed-off-by: Robert Jarzmik <robert.jarzmik@free.fr>
Reviewed-on: http://openocd.zylin.com/1448
Reviewed-by: Andreas Fritiofson <andreas.fritiofson@gmail.com>
Tested-by: jenkins