iio: adc: rcar-gyroadc: Enable compile-testing on non-ARM

The rcar-gyroadc driver compiles fine on other platforms, hence increase
compile coverage.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Simon Horman <horms+renesas@verge.net.au>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>