arm64: crypto: reduce priority of core AES cipher

The asynchronous, merged implementations of AES in CBC, CTR and XTS
modes are preferred when available (i.e., when instantiating ablkciphers
explicitly). However, the synchronous core AES cipher combined with the
generic CBC mode implementation will produce a 'cbc(aes)' blkcipher that
is callable asynchronously as well. To prevent this implementation from
being used when the accelerated asynchronous implemenation is also
available, lower its priority to 250 (i.e., below the asynchronous
module's priority of 300).

Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>