ARM: dts: sun8i: V3/V3s/S3/S3L: add pinctrl for UART2 RX/TX

The UART2 RX/TX pins on Allwinner V3 series is at PB0/1, which is used
as debugging UART on some boards.

Add pinctrl node for them.

Signed-off-by: Icenowy Zheng <icenowy@aosc.io>
Signed-off-by: Maxime Ripard <maxime@cerno.tech>
Link: https://lore.kernel.org/r/20200923005858.148261-1-icenowy@aosc.io