Make lpc_interrupt() attribute const

It's a placeholder at the moment, which makes it easy to say "this never
gives a different answer". It also silences our one compiler warning:

hw/lpc.c: In function 'lpc_interrupt':
hw/lpc.c:457:6: warning: function might be candidate for attribute 'const' [-Wsuggest-attribute=const]

Signed-off-by: Stewart Smith <stewart@linux.vnet.ibm.com>
abort if device tree parsing fails

I debugged a checkstop with a BML boot which turned out to be device
tree issues. A corrupt device tree is very likely to result in a
later checkstop, so abort in dt_expand to make debugging much easier.

Signed-off-by: Anton Blanchard <anton@samba.org>
Signed-off-by: Stewart Smith <stewart@linux.vnet.ibm.com>