cortex_m: fix stepping on FPB rev 1

cortex_m: fix stepping on FPB rev 1

Stepping in the maskisr auto mode sets breakpoint to step over interrupt
service tasks. If the device has FPB rev 1, setting hard breakpoint
is impossible on address over 0x1fffffff.

Use soft type breakpoint for adresses over 0x1fffffff if FPB is rev 1.
This may eventually fail if the code memory is not writeable, but there
is nothing to do in such case.

Change-Id: Ibdeeb506903a35d550b64f82c24c37a668de62b3
Signed-off-by: Tomas Vanek <vanekt@fbl.cz>
Reviewed-on: http://openocd.zylin.com/4857
Tested-by: jenkins
Reviewed-by: Antonio Borneo <borneo.antonio@gmail.com>