If of_state_fixup() fails for some reason, boot/bootm will fail, too

However, it is completely unclear where the error has happened, and to
the user it rather seems like the kernel device tree loaded from some
file was broken/corrupt, not that in fact some fixup routine was
failing.

Adding a meaningful error message can help debugging significantly.

Signed-off-by: Harald Welte <laforge@gnumonks.org>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>