ARM: dts: r8a7793: Add BRG support for SCIF

ARM: dts: r8a7793: Add BRG support for SCIF

Add the device node for the external SCIF_CLK.
The presence of the SCIF_CLK crystal and its clock frequency depends on
the actual board.

Add the two optional clock sources (ZS_CLK and SCIF_CLK for the internal
resp. external clock) for the Baud Rate Generator for External Clock
(BRG) to all SCIF and HSCIF device nodes.

This increases the range and accuracy of supported baud rates on
(H)SCIF.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>