dts: sun8i-h3: correct UART3 pin definitions

In a previous commit, I made a copy/paste error in the pinmux
definitions of UART3: PG{13,14} instead of PA{13,14}. This commit takes
care of that. I have tested this commit on Orange Pi PC and Orange Pi
Plus, and it works for these boards.

Fixes: e3d11d3c

 ("dts: sun8i-h3: add pinmux definitions for
UART2-3")

Signed-off-by: Jorik Jonker <jorik@kippendief.biz>
Acked-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Signed-off-by: Olof Johansson <olof@lixom.net>