at91sam3XXX.cfg: Configure JTAG clock to 500 kHz.

This affects all configurations including target/at91sam3XXX.cfg

Change-Id: I2c1e1edf0986d30e63f109604a38bf402ded369e
Signed-off-by: Ulf Samuelsson <ulf@emagii.com>
Reviewed-on: http://openocd.zylin.com/292
Tested-by: jenkins
Reviewed-by: Spencer Oliver <spen@spen-soft.co.uk>

tcl/target/at91sam3XXX.cfg

@@ -29,6 +29,18 @@ if { [info exists ENDIAN] } {
    set _ENDIAN little
 }
 
+# JTAG speed should be <= F_CPU/6. F_CPU after reset is 4 MHz, so use F_JTAG = 0.5MHz
+#
+# Since we may be running of an RC oscilator, we crank down the speed a
+# bit more to be on the safe side. Perhaps superstition, but if are
+# running off a crystal, we can run closer to the limit. Note
+# that there can be a pretty wide band where things are more or less stable.
+
+adapter_khz 500
+
+adapter_nsrst_delay 100
+jtag_ntrst_delay 100
+
 #jtag scan chain
 if { [info exists CPUTAPID] } {
    set _CPUTAPID $CPUTAPID