arm64: dts: renesas: eagle: Add SCIF_CLK support

Describe the external SCIF clock crystal, which can be used as a clock
source for the (High Speed) Serial Communication Interfaces with FIFO.

This improves serial console accuracy from 115200-257 bps to
115200+0 bps.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Link: https://lore.kernel.org/r/7dddaa362945118deab534ccfddfc0870abe8526.1673271243.git.geert+renesas@glider.be