arm64: dts: salvator-x: SCIF1 supports RTS/CTS hardware flow control

On the Salvator-X development board, the RTS and CTS pins of debug
serial-1 port SCIF1 are wired to the CP2102 Serial-USB bridge.  Reflect
this in the DTS by adding the "uart-has-rtscts" property to the scif1
device node.

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