ut: fix fdt_getprop_u32() change

fdt_getprop_u32 is not exported and it's different than what the
unit test uses. Rename u32 prop access methods to something that's
unit test specific.

Signed-off-by: Pantelis Antoniou <pantelis.antoniou@konsulko.com>
Acked-by: Simon Glass <sjg@chromium.org>