Remove output buffer limitation for PSA with GCM.

The requirement of minimum 15 bytes for output buffer in
psa_aead_finish() and psa_aead_verify() does not apply
to the built-in implementation of the GCM.

Alternative implementations are expected to verify the
length of the provided output buffers and to return
the MBEDTLS_ERR_GCM_BUFFER_TOO_SMALL in case the
buffer length is too small.

Signed-off-by: Mateusz Starzyk <mateusz.starzyk@mobica.com>

include/mbedtls/gcm.h

@@ -45,6 +45,8 @@
 #define MBEDTLS_ERR_GCM_AUTH_FAILED                       -0x0012
 /** Bad input parameters to function. */
 #define MBEDTLS_ERR_GCM_BAD_INPUT                         -0x0014
+/** An output buffer is too small. */
+#define MBEDTLS_ERR_GCM_BUFFER_TOO_SMALL                  -0x0018
 
 #ifdef __cplusplus
 extern "C" {