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.

Acked-by: Nicolas Pitre <nico@linaro.org>

Change-Id: I8e924dac2ad199f1024ae04b03f7e136149dd394
Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>