Use startup file and linker script from TF-M

The trusted-firmware-m repository provides an NS (non-secure) startup
file and linker script for each platform. We switch the TF-M example to
use those.

The revision of TF-M has been bumped to a newer version with the fixes
we need:
* 61483155 platform: Fix missing PSP initial value and limit
* a1352287 build: Make ARMClang maximum version check a warning

Signed-off-by: Lingkai Dong <lingkai.dong@arm.com>