Interface: Provide implementation of tfm_ns_interface_dispatch()

This patch provides implementation of tfm_ns_interface_dispatch for
bare metal and RTOS use cases. This is needed to simplify TF-M
integration with NS application.
For more details refer to
"Default implementation of tfm_ns_interface_dispatch()" mailing thread:
https://lists.trustedfirmware.org/archives/list/tf-m@lists.trustedfirmware.org/message/ZCBRUODVTBK26JGAZKKIJEQ62D3XIMSO/
and TF-M tech forum presentation
"Default implemeation of tsm_ns_interface_dispatch":
https://www.trustedfirmware.org/meetings/tf-m-technical-forum/



Following things are done:
 * tfm_ns_interface.c was moved from test repo to TF-M repo
   tfm_ns_interface_rtos.c file
 * tfm_ns_interface_bare_metal.c is added.
 * required NS OS wrapper header files are moved from test to TFM repo
 * tfm_ns_interface.c.example is removed as no longer needed
   are provided.
 * documentation is updated

Signed-off-by: Bohdan Hunko <Bohdan.Hunko@infineon.com>
Signed-off-by: Chris Brand <chris.brand@cypress.com>
Change-Id: I3e7fd80946bd674cb562ddac8fbc8c7053e30478