cortex_m: handle armv8m cores without security extension

cortex_m: handle armv8m cores without security extension

Cores armv8m, e.g. Cortex-M33, can be instantiated without the
optional Security Extension.
In this case, the secure registers are not present and when GDB
try accessing them it triggers a set of errors.

For armv8m cores without security extension, don't provide to GDB
the description of the secure registers.

Change-Id: I254478a4cf883e85b786df3f62c726b2f40d88d9
Signed-off-by: Antonio Borneo <borneo.antonio@gmail.com>
Reported-by: Torbjörn SVENSSON <torbjorn.svensson@foss.st.com>
Reviewed-on: https://review.openocd.org/c/openocd/+/7402
Tested-by: jenkins
Reviewed-by: Tomas Vanek <vanekt@fbl.cz>