suniv: add UART1 support

Some boards with the Allwinner F1C100s family SoCs use UART1 for its
debug UART, so define the pins for the SPL and the pinmux name and mux
value for U-Boot proper.

Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Reviewed-by: Jernej Skrabec <jernej.skrabec@gmail.com>