ARM: dts: sun8i: r40: Add pinmux setting for CLK_OUT_A

CLK_OUT_A, an external clock output function driven from the clock
control unit, on the R40 is sometimes used to provide a low rate low
power clock to a WiFi or Bluetooth controller.

This patch adds a pinmux setting for it.

Signed-off-by: Chen-Yu Tsai <wens@csie.org>
Signed-off-by: Maxime Ripard <maxime.ripard@bootlin.com>