arm64: dts: renesas: r8a77990: Add BRG support to SCIF2

Add the device node for the external SCIF_CLK, and describe the clock
inputs for the Baud Rate Generator for External Clock (BRG) for SCIF2,
which can increase serial clock accuracy.

The presence of the SCIF_CLK crystal and its clock frequency depend on
the actual board.

Signed-off-by: Takeshi Kihara <takeshi.kihara.df@renesas.com>
[geert: Enhance patch description]
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>