arm: mvebu: Introduce mvebu-u-boot.dtsi for 32-bit Armada SoCs

Set u-boot,dm-pre-reloc for /soc/, /soc/internal-regs/ and &uart0 nodes as
it is required on every 32-bit Armada SoCs. And set also u-boot,dm-pre-reloc
for &spi0 when going to boot from SPI because otherwise SPL SPI drivers do
not load.

Signed-off-by: Pali Rohár <pali@kernel.org>
Reviewed-by: Stefan Roese <sr@denx.de>
Tested-by: Stefan Roese <sr@denx.de>