ARM64: dts: meson-gx: add the serial CTS and RTS pin groups

This adds pinctrl group nodes for the CTS and RTS pins of each serial
controller. This makes it possible to enable the CTS and RTS pins which
are controlled by the serial controller hardware (through the meson_uart
driver).

Signed-off-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
Tested-by: Andreas Färber <afaerber@suse.de>
Signed-off-by: Kevin Hilman <khilman@baylibre.com>