interface: define TMS sequence command

For support of SWD we need to be able to clock out special bit
sequences over TMS or SWDIO.  Create this as a generic operation,
not yet called by anything, which is split as usual into:

 - upper level abstraction ... here, jtag_add_tms_seq();
 - midlayer implementation logic hooking that to the lowlevel code;
 - lowlevel minidriver operation ... here, interface_add_tms_seq();
 - message type for request queue, here JTAG_TMS.

This is done slightly differently than other operations: there's a flag
saying whether the interface driver supports this request.  (In fact a
flag *word* so upper layers can learn about other capabilities too ...
for example, supporting SWD operations.)

That approach (flag) lets this method *eventually* be used to eliminate
pathmove() and statemove() support from most adapter drivers, by moving
all that logic into the mid-layer and increasing uniformity between the
various drivers.  (Which will in turn reduce subtle bugginess.)

Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>

src/jtag/interface.h

@@ -184,10 +184,28 @@ static inline tap_state_t jtag_debug_state_machine(const void *tms_buf,
 }
 #endif // _DEBUG_JTAG_IO_
 
+/**
+ * Represents a driver for a debugging interface.
+ *
+ * @todo Rename; perhaps "debug_driver".  This isn't an interface,
+ * it's a driver!  Also, not all drivers support JTAG.
+ *
+ * @todo We need a per-instance structure too, and changes to pass
+ * that structure to the driver.  Instances can for example be in
+ * either SWD or JTAG modes.  This will help remove globals, and
+ * eventually to cope with systems which have more than one such
+ * debugging interface.
+ */
 struct jtag_interface {
 	/// The name of the JTAG interface driver.
 	char* name;
 
+	/**
+	 * Bit vector listing capabilities exposed by this driver.
+	 */
+	unsigned supported;
+#define DEBUG_CAP_TMS_SEQ	(1 << 0)
+
 	/**
 	 * Execute queued commands.
 	 * @returns ERROR_OK on success, or an error code on failure.