crypto: arm/speck - fix building in Thumb2 mode

Building the kernel with CONFIG_THUMB2_KERNEL=y and
CONFIG_CRYPTO_SPECK_NEON set fails with the following errors:

    arch/arm/crypto/speck-neon-core.S: Assembler messages:

    arch/arm/crypto/speck-neon-core.S:419: Error: r13 not allowed here -- `bic sp,#0xf'
    arch/arm/crypto/speck-neon-core.S:423: Error: r13 not allowed here -- `bic sp,#0xf'
    arch/arm/crypto/speck-neon-core.S:427: Error: r13 not allowed here -- `bic sp,#0xf'
    arch/arm/crypto/speck-neon-core.S:431: Error: r13 not allowed here -- `bic sp,#0xf'

The problem is that the 'bic' instruction can't operate on the 'sp'
register in Thumb2 mode.  Fix it by using a temporary register.  This
isn't in the main loop, so the performance difference is negligible.
This also matches what aes-neonbs-core.S does.

Reported-by: Stefan Agner <stefan@agner.ch>
Fixes: ede96221

 ("crypto: arm/speck - add NEON-accelerated implementation of Speck-XTS")
Signed-off-by: Eric Biggers <ebiggers@google.com>
Acked-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Reviewed-by: Stefan Agner <stefan@agner.ch>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>