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