boards: st: b_u585_iot02a: stick to RSA signature type in TF-M's MCUboot

The default has been changed to EC-P256, but there are issues with
this board when TF-M is compiled with MCUBOOT_USE_PSA_CRYPTO (required
by EC-P256) because of its integration in TF-M.
Do not enable PSA crypto on this board for the time being.

Signed-off-by: Tomi Fontanilles <tomi.fontanilles@nordicsemi.no>