target/arm720t: fix memory leak of register cache

target/arm720t: fix memory leak of register cache

There is no method to free the register cache, allocated in
arm720t_init_target().
Issue identified by tracking all calls to arm7tdmi_init_target().

Implement the method arm720t_deinit_target() by calling directly
arm7tdmi_deinit_target().

NOT TESTED on a real arm720t target.
Tested on a arm926ejs (SPEAr320) by hacking the target type and
pretending it is a xscale:
	sed -i s/arm926ejs/arm720t/ tcl/target/spear3xx.cfg

Change-Id: I53c1f46c1a355a710e8df01468b19220671569dc
Signed-off-by: Antonio Borneo <borneo.antonio@gmail.com>
Reviewed-on: http://openocd.zylin.com/5697
Tested-by: jenkins