aw_nmi: add support for a31/a83t's r_intc

We currently support the a83t's r_intc in a somewhat hack-ish way; our .dts
describes it as nmi_intc, and uses a subset of the actual register space to
make it line up with a20/a31 nmi offsets.

This breaks with the recent 4.14 update describing r_intc using the full
register space, so update aw_nmi to use the correct register offsets with
the right compat data in a way that doesn't break our current dts with
nmi_intc or upstream with r_intc described.

Reviewed by: manu
Approved by: emaste (mentor)
Differential Revision: https://reviews.freebsd.org/D13122