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 (S1 and SCIF_CLK for the internal
resp. external clock) for the Baud Rate Generator for External Clock
(BRG) to all SCIF device nodes.

This increases the range and accuracy of supported baud rates on SCIF.
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>