ARM: dts: sun8i: r40: Add clock accuracy for external oscillators

The R40 datasheet specifies a tolerance range for the external
oscillators used. Add them to the device tree as the clock accuracy.

Acked-by: Maxime Ripard <maxime.ripard@bootlin.com>
Tested-by: Corentin Labbe <clabbe.montjoie@gmail.com>
Signed-off-by: Chen-Yu Tsai <wens@csie.org>