target/arm926ejs: fix memory leaks

target/arm926ejs: fix memory leaks

The memory leaks detected and fixed are:
- arm register cache;
- EmbeddedICE register cache;
- arm_jtag_reset_callback internal data;
- struct arm926ejs_common.

Issue identified with valgrind.
Tested on SPEAr320 based on arm926ejs.

Change-Id: If2bed02c516051ce4d0eb29b204a3f3337fe5d6a
Signed-off-by: Antonio Borneo <borneo.antonio@gmail.com>
Reviewed-on: http://openocd.zylin.com/5698
Tested-by: jenkins