flash: ROM support

The only reason to define a read-only region is that gdb needs
a complete memory map to choose hard or soft breakpoints properly.

Change-Id: I9d05cb6b91f054ad5cc9333af6b14eb433dbdc99
Signed-off-by: Tomas Vanek <vanekt@fbl.cz>
Reviewed-on: https://review.openocd.org/c/openocd/+/5106
Reviewed-by: Antonio Borneo <borneo.antonio@gmail.com>
Tested-by: jenkins

doc/openocd.texi

@@ -6178,6 +6178,16 @@ flash bank vbank1 virtual 0x9fc00000 0 0 0 \
 @end example
 @end deffn
 
+@deffn {Flash Driver} read_only
+A stub driver without write and erase.
+Use to define a ROM region for the gdb memory map.
+
+@example
+flash bank $_CHIPNAME.sysrom read_only 0x1ff00000 0xedc0 0 0 \
+           $_TARGETNAME
+@end example
+@end deffn
+
 @subsection External Flash
 
 @deffn {Flash Driver} {cfi}