ARM: dts: sun8i: add pinmux for UART1 at PG

The UART1 at PG (PG6, PG7, PG8, PG9) is, in the Allwinner's reference
tablet design of A23/33, used to connect to UART Bluetooth cards.

Add the pinmux for it.

Signed-off-by: Icenowy Zheng <icenowy@aosc.xyz>
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>