fdt: adjust bcm283x device tree for u-boot

The information currently set via platdata has to be represented in the
device tree now. bcm283x-uboot.dtsi adds the u-boot specific "skip-init"
property to the serial nodes and enables initialization in the pre-reloc phase.

Cc: Albert Aribaud <albert.u.boot@aribaud.net>
Signed-off-by: Fabian Vogt <fvogt@suse.com>
Reviewed-by: Simon Glass <sjg@chromium.org>