ARM: sunxi: cubieboard: Add UART muxing

We previously relied on the bootloader to do the muxing of the UART for
the Cubieboard. Don't rely on it anymore and use pinctrl.

Also remove uart1, as it is not enabled by default and it's not exposed
on the board headers.

Signed-off-by: Emilio López <emilio@elopez.com.ar>
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>