microblaze: Sane handling of missing timer/intc in device tree

This code path doesn't test any returned pointers for NULL, leading to a bad
kernel page fault if there's no timer/intc found.

Slightly better is to BUG(), but even better still would be a printk beforehand.

Signed-off-by: John Williams <john.williams@petalogix.com>
Signed-off-by: Michal Simek <monstr@monstr.eu>