Looking at - https://git.trustedfirmware.org/trusted-firmware-m.git/tree/app/main_ns.c#n90
tfm_ns_wait_for_s_cpu_ready() -> tfm_platform_ns_wait_for_s_cpu_ready() return a PLATFORM_MAILBOX_SUCCESS which seems to be very low level code, shouldn't it be TFM_SUCCESS ?
There may be some platforms other than PSoC6, which doesn't require tfm_ns_wait_for_s_cpu_ready() implementation. PLATFORM_MAILBOX_SUCCESS is a low-level error code and should be replaced by TFM_SUCCESS.