ARM: sunxi: hackberry: Add UART muxing

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

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