target/ti-cjtag: make switching to JTAG more reliable

The current cJTAG to JTAG switching commands for TI chips are not
particularly reliable, especially on chips with accurate timing.
On a Raspberry Pi the existing sequence has (depending on cabling and
chip) a ~50% chance of working, on a much better-behaved FT2232H
it doesn't manage to enable full JTAG at all.

This change runs a bunch of test-idle cycles before actually attempting
to switch to full JTAG. This makes the switch reliable even at high
clock speeds (>100kHz) and from precise sources like the FT2232H.

Change-Id: I9293e884bf3e9606d529756ae4483b844d3c39db
Reported-by: Phil Wiggum <p1mail2015@mail.com>
Fixes: https://sourceforge.net/p/openocd/tickets/375/
Signed-off-by: Lorenz Brun <lorenz@brun.one>
Reviewed-on: https://review.openocd.org/c/openocd/+/7419
Tested-by: jenkins
Reviewed-by: Antonio Borneo <borneo.antonio@gmail.com>

tcl/target/ti-cjtag.cfg

@@ -5,6 +5,7 @@
 # Read section 6.3 in http://www.ti.com/lit/pdf/swru319 for more information.
 proc ti_cjtag_to_4pin_jtag {jrc} {
 	# Bypass
+	runtest 20
 	irscan $jrc 0x3f -endstate RUN/IDLE
 	# Two zero bit scans and a one bit drshift
 	pathmove RUN/IDLE DRSELECT DRCAPTURE DREXIT1 DRPAUSE