ARM: dts: sun8i: ippo-q8h: Add pinctrl properties for R_UART

Now that we have R_PIO controller support and the pinmux for R_UART,
add the correct pinctrl properties to the R_UART node.

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