crypto: arm/aes-neonbs - resolve fallback cipher at runtime

Currently, the bit sliced NEON AES code for ARM has a link time
dependency on the scalar ARM asm implementation, which it uses as a
fallback to perform CBC encryption and the encryption of the initial
XTS tweak.

The bit sliced NEON code is both fast and time invariant, which makes
it a reasonable default on hardware that supports it. However, the
ARM asm code it pulls in is not time invariant, and due to the way it
is linked in, cannot be overridden by the new generic time invariant
driver. In fact, it will not be used at all, given that the ARM asm
code registers itself as a cipher with a priority that exceeds the
priority of the fixed time cipher.

So remove the link time dependency, and allocate the fallback cipher
via the crypto API. Note that this requires this driver's module_init
call to be replaced with late_initcall, so that the (possibly generic)
fallback cipher is guaranteed to be available when the builtin test
is performed at registration time.

Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>