samples: tfm_ipc: call sys_arch_reboot(..) directly

Switch to calling the sys_arch_reboot() API directly,
instead of going through the generic sys_reboot API.
This is to avoid locking the IRQs before the reset is
called, which is breaking the PSA call execution.

Also, align sample code to use the same secure service
(platform reset) to show case the PSA connect and close
APIs, instead of a random one (since the reset service
has to work to ultimately perform the system reset).

Signed-off-by: Ioannis Glaropoulos <Ioannis.Glaropoulos@nordicsemi.no>