Platform/RPi3: Report device tree modification errors using Print()

Most functions in FdtDxe currently return VOID and report errors using
only DEBUG.  Update functions to return EFI_STATUS and report errors
using Print() so that errors are at least visible in non-debug builds.

Signed-off-by: Michael Brown <mbrown@fensystems.co.uk>
Reviewed-by: Leif Lindholm <leif.lindholm@linaro.org>