target/cortex_a: fix memory leak of register cache

There is no method to free the register cache, allocated in
armv4_5, so we get a memory leak.
Issue identified by valgrind.

Implement the method arm_free_reg_cache() and call it in cortex_a
deinit and to exit for error during arm_dpm_setup().
Tested on dual cortex-A stm32mp15x.
This change is inspired from similar fix in commit b01b5fe13a
("armv7m: Fix memory leak in register caching.").

The same allocation is also used by target types "arm7tdmi",
"arm9tdmi", "arm11" and "xscale" but they all lack the deinit
method and I do not have relevant HW to test the fix. For such
reasons they are not addressed in this patch.

Change-Id: I4da1e1f12e36ec245d1f3b11a4eafcbd9a1d2e25
Signed-off-by: Antonio Borneo <borneo.antonio@gmail.com>
Reviewed-on: http://openocd.zylin.com/5693
Tested-by: jenkins

src/target/arm.h

@@ -272,6 +272,8 @@ struct arm_reg {
 };
 
 struct reg_cache *arm_build_reg_cache(struct target *target, struct arm *arm);
+void arm_free_reg_cache(struct arm *arm);
+
 struct reg_cache *armv8_build_reg_cache(struct target *target);
 
 extern const struct command_registration arm_command_handlers[];