ARM: crypto: add NEON accelerated XOR implementation

Add a source file xor-neon.c (which is really just the reference
C implementation passed through the GCC vectorizer) and hook it
up to the XOR framework.

Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Acked-by: Nicolas Pitre <nico@linaro.org>

Change-Id: Iaf799f7661a3249276b7040bce1b26ebc1e99397