ML-DSA: no keys on the stack in low-stack mode

[gilles-peskine-arm]

When ML-DSA is configured in low-stack mode (TF_PSA_CRYPTO_PQCP_BUFFER_ALLOC introduced by #786), also use the global buffer instead of a large stack variable (~4kB) for temporary copies of keys.

Also, in signature, when the expanded private key is available (#774), make sure we never use a large stack variable or a heap variable.

Prerequisites:

• Configuration option for using a global variable as the ML-DSA workspace #645
• Support expanded key format in ML-DSA signature driver #774 for the additional bit about signature.