arm64: dts: allwinner: a64: Add UART2 pin nodes

UART2 is exposed on the Pi connector of Pine64. Make a pinctrl node
available at the SoC level, to simplify enabling UART2 via DT overlay.

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