sunxi: support boot console on uart1 for sun8i

The A23, A33, H3, H5, A83T, V3 and Sochip S3 sun8i SoCs can mux uart1 on
GPIOs PG6 and PG7. This patch adds support for using uart1 on those pins
as boot console.

Signed-off-by: Tobias Schramm <t.schramm@manjaro.org>
Reviewed-by: Andre Przywara <andre.przywara@arm.com>
Signed-off-by: Andre Przywara <andre.przywara@arm.com>