jtag/drivers/openjtag: Add support for Cypress CY7C65215

The Cypress CY7C65215 Dual Channel USB-Serial Bridge Controller [1]
understands the OpenJTAG protocol over a proprietary USB interface.

This patch adds support for the CY7C65215 to the openjtag interface
driver.  A new configuration option, `openjtag_variant`, allows to
select the transport to use.

Libusb (1.x or 0.1) is now a hard dependency of the openjtag driver.
This should not be a big issue as libftdi also depends on it.

[1] http://www.cypress.com/?rID=82870

Change-Id: I55ffb3fd9e006eb311e405d9fb836bb119644bfd
Signed-off-by: Vianney le Clément de Saint-Marcq <vianney.leclement@essensium.com>
Signed-off-by: Ricardo Ribalda Delgado <ricardo.ribalda@gmail.com>
Reviewed-on: http://openocd.zylin.com/2805
Tested-by: jenkins
Reviewed-by: Jiri Kastner <cz172638@gmail.com>
Reviewed-by: Paul Fertser <fercerpav@gmail.com>

doc/openocd.texi

@@ -2912,6 +2912,27 @@ pinout.
 
 @end deffn
 
+@deffn {Interface Driver} {openjtag}
+OpenJTAG compatible USB adapter.
+This defines some driver-specific commands:
+
+@deffn {Config Command} {openjtag_variant} variant
+Specifies the variant of the OpenJTAG adapter (see @uref{http://www.openjtag.org/}).
+Currently valid @var{variant} values include:
+
+@itemize @minus
+@item @b{standard} Standard variant (default).
+@item @b{cy7c65215} Cypress CY7C65215 Dual Channel USB-Serial Bridge Controller
+(see @uref{http://www.cypress.com/?rID=82870}).
+@end itemize
+@end deffn
+
+@deffn {Config Command} {openjtag_device_desc} string
+The USB device description string of the adapter.
+This value is only used with the standard variant.
+@end deffn
+@end deffn
+
 @section Transport Configuration
 @cindex Transport
 As noted earlier, depending on the version of OpenOCD you use,