Describe the second debug serial port (CN9800) on the Gray Hawk Single
board, as provided by HSCIF2, including the SCIF_CLK2 external clock
source, and all related pin control.

Based on a patch for Gray Hawk in the BSP by Nghia Nguyen.
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Link: https://lore.kernel.org/r/6e1faff6a717cb8344661bafcae5db5dcfb53a90.1709741303.git.geert+renesas@glider.be