target/stm32h7x: Fix documentation of reset_config

The stm32h7x.cfg does not specify connect_assert_srst or
connect_deassert_srst in its reset_config. The comment claims that it
will therefore connect in reset. However, per the manual, the default
configuration is actually connect_deassert_srst, not
connect_assert_srst. In actual fact, connect_assert_srst does not work
on the STM32H7 because, while SRST is asserted, everything on the AXI
bus is inaccessible. The CPU core is accessible, but since the
examine-end event handler also pokes at the DBGMCU peripheral, that will
fail in connect_assert_srst mode. So using connect_deassert_srst is
appropriate, so fix the comment accordingly.

Change-Id: If3e32e871fb19cc61183bdf911b7c5efd80b62e2
Signed-off-by: Christopher Head <chead@zaber.com>
Reviewed-on: http://openocd.zylin.com/4741
Tested-by: jenkins
Reviewed-by: Tomas Vanek <vanekt@fbl.cz>

tcl/target/stm32h7x.cfg

@@ -56,7 +56,17 @@ if {[using_jtag]} {
  jtag_ntrst_delay 100
 }
 
-# use hardware reset, connect under reset
+# use hardware reset
+#
+# The STM32H7 does not support connect_assert_srst mode because the AXI is
+# unavailable while SRST is asserted, and that is used to access the DBGMCU
+# component at 0x5C001000 in the examine-end event handler.
+#
+# It is possible to access the DBGMCU component at 0xE00E1000 via AP2 instead
+# of the default AP0, and that works with SRST asserted; however, nonzero AP
+# usage does not work with HLA, so is not done by default. That change could be
+# made in a local configuration file if connect_assert_srst mode is needed for
+# a specific application and a non-HLA adapter is in use.
 reset_config srst_only srst_nogate
 
 if {![using_hla]} {