freertos-ota-pal-psa: Use the RSA 3072 bit public key

Currently RSA 2048 bit public key is used to validate the incoming OTA
update signature. With latest tf-m, mcuboot is using RSA 3072 bit keys.
Therefore, use public key derived from root-RSA-3072_1.pem.

Signed-off-by: Devaraj Ranganna <devaraj.ranganna@arm.com>