ARM: dts: kzm9g: SCIFA4 supports RTS/CTS hardware flow control

On the KZM-A9-GT development board, the RTS and CTS pins of the debug
console SCIFA4 are wired to the CP2102 Serial-USB bridge.  Reflect
this in the DTS by adding the "uart-has-rtscts" property to the scifa4
device node.

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