arm64: dts: allwinner: pine64: Prepare optional UART nodes with pinctrl

Pine64 exposes all A64 UARTs, not just UART0.

Since the pins can be used as GPIO, don't enable the new UART nodes by
default, but prepare the pinctrl settings to aid in activating them via
overlays, i.e., overriding the status property of &uartX nodes.

For UART4 (Euler) the safer route of not including RTS/CTS pins is chosen,
whereas for UART1 (Bluetooth) they are included.

Add the corresponding pinctrl nodes where missing.

Suggested-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Signed-off-by: Andreas Färber <afaerber@suse.de>
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>