arm64: dts: renesas: white-hawk-cpu: Add serial port pin control

Complete the description of the serial console and the external serial
clock by adding pin control.

Based on larger patch in the BSP by Takeshi Kihara.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Link: https://lore.kernel.org/r/74c862ef6e46b4af398d9b371ff38fae17b3db05.1662715538.git.geert+renesas@glider.be