libfdt: Correct prototype for fdt_ro_probe_()

This function returns an int32_t, however the prototype in
libfdt_internal.h shows it returning an int.  We haven't caught this before
because they're the same type on nearly all platforms this gets built on.
Apparently it's not the case on FreeRTOS, so someone hit this mismatch
building for that platform.

Reported-by: dharani kumar <dharanikumarsrvn@gmail.com>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>