Limit keys to 65528 bits

65528 bits is more than any reasonable key until we start supporting
post-quantum cryptography.

This limit is chosen to allow bit-sizes to be stored in 16 bits, with
65535 left to indicate an invalid value. It's a whole number of bytes,
which facilitates some calculations, in particular allowing a key of
exactly PSA_CRYPTO_MAX_STORAGE_SIZE to be created but not one bit
more.

As a resource usage limit, this is arguably too large, but that's out
of scope of the current commit.

Test that key import, generation and derivation reject overly large
sizes.

include/psa/crypto_struct.h

@@ -309,6 +309,12 @@ static inline struct psa_key_policy_s psa_key_policy_init( void )
     return( v );
 }
 
+/* The maximum size of a key in bits.
+ * This is a whole number of bytes, to facilitate some calculations
+ * such as the maximum size of key data in storage.
+ */
+#define PSA_MAX_KEY_BITS 0xfff8
+
 typedef struct
 {
     psa_key_type_t type;