cmd/fdt.c align data buffer to avoid unaligned word access

Since the compiler is free to place a char array to any address in
memory (in this case the stack), also to a non word aligned address the
function "fdt_prop_parse" runs into troubles upon it wants to write some
(fdt32_t *) to such a variable (if it has been placed to a none word
aligned address).

To avoid this we tell the compiler to always align this scratchpad to a
word aligned address.

Signed-off-by: Bernhard Messerklinger <bernhard.messerklinger@br-automation.com>
Reviewed-by: Hannes Schmelzer <oe5hpm@oevsv.at>
Tested-by: Hannes Schmelzer <oe5hpm@oevsv.at>
Reviewed-by: Simon Glass <sjg@chromium.org>