Core: Support level 2 in scheduler

- Move tfm_pendsv_do_schedule() from tfm_thread.c to tfm_spm.c.
- Add isolation level 2 support in tfm_pendsv_do_schedule().

Change-Id: Ida37cd9210710c22a7d336ac8041556eb6a6dba3
Signed-off-by: Ken Liu <ken.liu@arm.com>