psa: config: Add MBEDTLS_PSA_CRYPTO_SPM

We failed check-names.sh due to using a define which wasn't described or
defined anywhere. Even though we won't realistically enable
MBEDTLS_PSA_CRYPTO_SPM via the configuration system (and will do it from
PSA Crypto SPM tooling instead), add a description of the configuration to
config.h as good practice. Exclude MBEDTLS_PSA_CRYPTO_SPM from the "full"
configuration as well.

configs/config-psa-crypto.h

@@ -696,6 +696,20 @@
  */
 #define MBEDTLS_PKCS1_V21
 
+/**
+ * \def MBEDTLS_PSA_CRYPTO_SPM
+ *
+ * When MBEDTLS_PSA_CRYPTO_SPM is defined, the code is built for SPM (Secure
+ * Partition Manager) integration which separates the code into two parts: a
+ * NSPE (Non-Secure Process Environment) and an SPE (Secure Process
+ * Environment).
+ *
+ * Module:  library/psa_crypto.c
+ * Requires: MBEDTLS_PSA_CRYPTO_C
+ *
+ */
+//#define MBEDTLS_PSA_CRYPTO_SPM
+
 /**
  * \def MBEDTLS_RSA_NO_CRT
  *