[crypto] Align PSA P256Keypair::Serialize buffer cleanup with the mbedTLS backend (project-chip#73344)

The mbedTLS implementation of P256Keypair::Serialize calls ClearSecretData
on its temporary buffer before returning. Apply the same call in the two
PSA-based implementations so all three follow one convention.

Author: Alami-Amine

src/crypto/CHIPCryptoPALPSA.cpp

@@ -882,6 +882,7 @@ CHIP_ERROR P256Keypair::Serialize(P256SerializedKeypair & output) const
     error = output.SetLength(bbuf.Needed());
 
 exit:
+    ClearSecretData(privateKey, sizeof(privateKey));
     LogPsaError(status);
 
     return error;

src/platform/silabs/efr32/CHIPCryptoPALPsaEfr32.cpp

@@ -849,6 +849,7 @@ CHIP_ERROR P256Keypair::Serialize(P256SerializedKeypair & output) const
     error = output.SetLength(bbuf.Needed());
 
 exit:
+    ClearSecretData(privateKey, sizeof(privateKey));
     LogPsaError(status);
 
     return error;