ARM: dts: sun8i: r40: Add pinmux settings for UART3 on PG pingroup

UART3 on the PG pingroup on the R40 SoC is commonly used to connect the
bluetooth controller in a WiFi+Bluetooth combo chip, with the WiFi bits
also on the PG pingroup.

This patch adds two device nodes for UART3 on PG pingroup, one for the
RX/TX pins, and one for the RTS/CTS pins. Consumers can reference either
just the RX/TX pinmux setting or both, depending on the application.

Signed-off-by: Chen-Yu Tsai <wens@csie.org>
Signed-off-by: Maxime Ripard <maxime.ripard@bootlin.com>