PK: don't use mbedtls_ecp_check_pub_priv() when USE_PSA is enabled.

[mpg]

Context: see #6839; this is a step towards ECPa.PK.

Using mbedtls_ecp_check_pub_priv() is a dependency on ECP arithmetic which we're trying to avoid. Let's try another approach.

For example, now we can call mbedtls_pk_write_pubkey() on both prv and pub and compare the results - edit: or we could directly got something more future-proof, see below.

Notes for the future:

• When Avoid parse/unparse public ECC keys in PK with USE_PSA #7202 is done, we can call it only on prv and directly compare to the bytes array stored in pub.
• When private key are done too, we can call psa_import() + psa_export_public() on prv and compare to the bytes array stored in pub.

Prototype: #7357

[mpg]

since also for the key creation we are already importing the private key into PSA to get its public part, why can't we go that way directly also here? In theory also 7202 is a sort of middle step toward having everything on the PSA side (or at least there is a discussion on this)

Indeed, instead of doing something that we'll then change when 7202 and 7074 are done, we should go directly for something closer to the final version.

Also, it might create an opportunity for saving code by sharing it with pkparse.