target/armv7a_cache: add gdb keep-alive and fix a missing dpm finish

target/armv7a_cache: add gdb keep-alive and fix a missing dpm finish

Depending on range size, the loop on cache operations can take quite
some time, causing gdb to timeout.

Add keep-alive to prevent gdb to timeout.
Add also a missing dpm->finish() to balance dpm->prepare().

Change-Id: Ia87934b1ec19a0332bb50e3010b582381e5f3685
Signed-off-by: Antonio Borneo <borneo.antonio@gmail.com>
Reviewed-on: http://openocd.zylin.com/4627
Tested-by: jenkins
Reviewed-by: Matthias Welwarsky <matthias@welwarsky.de>