arm64: dts: renesas: gray-hawk-single: Add serial console pin control

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

Based on patches for Gray Hawk in the BSP by Hai Pham and Nghia Nguyen.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Link: https://lore.kernel.org/r/b1eb2d3364d5ead7f7bcf7a737c5914971db64d3.1706266286.git.geert+renesas@glider.be