powerpc/boot: Fixup device-tree on little endian

This fixes the core devtree.c functions and the ns16550 UART backend.

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Paul Mackerras <paulus@ozlabs.org>
Reviewed-by: Segher Boessenkool <segher@kernel.crashing.org>
Acked-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/YMwXrPT8nc4YUdJ9@thinks.paulus.ozlabs.org