ARM: sunxi: DT: Add stdout-path property

Add UART aliases and stdout-path property for all the Allwinner boards so that
we won't have to rely on the bootargs' console= value, while working with
legacy bootloaders.

While we're at it, also remove the mentions of earlyprintk in the bootargs,
that will remove our default bootargs entirely, and allow the kernel to boot on
a system even if DEBUG_LL is configured for another system.

Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>