am335x-sancloud-bbe: Add -u-boot.dtsi files

The SanCloud BBE requires the same dtb nodes to be present in the SPL as
the AM335x EVM.

The SanCloud BBE Lite also requires the SPI flash node and all
dependencies to be present in the SPL to support SPI boot.

Signed-off-by: Paul Barker <paul.barker@sancloud.com>