ARM: dts: sun8i: V3/S3: Add UART1 pin definitions to the V3/S3 dtsi

The Allwinner V3 and S3 can use PG6/7 as RX/TX for UART1. Since no other
functions are assigned to those pins, they are a convenient choice for
a debugging or application UART.
This is specific to V3/S3 as the V3s's non-BGA package did not have
those pins.

Signed-off-by: Matteo Scordino <matteo.scordino@gmail.com>
Signed-off-by: Maxime Ripard <maxime@cerno.tech>
Link: https://lore.kernel.org/r/20201030234325.5865-3-matteo.scordino@gmail.com