We can't build documentation when doing a cross build, so disable it
explicitly to be clear.
(From OE-Core rev: 2706062d3a711ff0b2c03bee40b908ad329ebd6d)
Signed-off-by: Ross Burton <ross.burton@arm.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>