ARM: 7825/1: document the use of NEON in kernel mode

Add a file to Documentation/arm explaining how kernel mode NEON
is supposed to be used.

Reviewed-by: Nicolas Pitre <nico@linaro.org>
Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>