AES-XTS support in PSA #6384

Suggested enhancement

I tried to use the PSA_ALG_XTS cipher with the psa library and ran into two issues:

1. I was not able to generate a PSA_KEY_TYPE_AES key for AES-256-XTS (bits = 512), psa_generate_key returns PSA_INVALID_ARGUMENT. It works with bits = 256 though i.e. AES-128-XTS.

2. Trying to encrypt with PSA_ALG_XTS and a 256 bit AES key returns PSA_NOT_SUPPORTED. And indeed the switch statement here doesn't seem to mention PSA_ALG_XTS or MBEDTLS_MODE_XTS.

Justification

Mbed TLS needs this because it already seems to have support for it.

Edit (mpg): we might need this for 4.0