message: [cmake] include CoAPS PSK support for OTBR
  Turn on mbedtls PSK support for CoAP testing on reference OTBR
  (similar to ECDSA).
diff: #define MBEDTLS_SSL_SRV_C
  #define MBEDTLS_SSL_TLS_C
  -// Enable ECDSA support
  +// Enable CoAPS PSK support
  +#define MBEDTLS_KEY_EXCHANGE_PSK_ENABLED
  +
  +// Enable CoAPS ECDSA support
  #define MBEDTLS_BASE64_C
  #define MBEDTLS_ECDH_C
  #define MBEDTLS_ECDSA_C
  