arm64: dts: renesas: spider: Complete SCIF3 description

Complete the description of the serial console by adding RTS/CTS, the
external clock crystal, and pin control.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Yoshihiro Shimoda <yoshihiro.shimoda.uh@renesas.com>
Link: https://lore.kernel.org/r/8e5701ca2a5f5925918217ab79e8489535339e7b.1645458249.git.geert+renesas@glider.be