arm64: Add a bus_describe_intr() implementation for nexus

[markj]

Summary

Prompted by a compiler warning introduced by
e582d4a2b09e ("arm64: nexus code tidy-up").

[mhorne]

Sure, but it should be done for riscv as well. This method is not widely used but we may as well have it.

sys/arm64/arm64/nexus.c

123

I see. This was a copy-paste error.

[markj]

In D39749#904580, @mhorne wrote:

Sure, but it should be done for riscv as well.

Ok.

This method is not widely used but we may as well have it.

On my test platform it's used only by nvme. Now vmstat -i shows:

its0,0: nvme0:admin                                      25          0
its0,1: nvme0:io0                                       439          8
its0,2: nvme0:io1                                       330          6
its0,3: nvme0:io2                                       452          8
its0,4: nvme0:io3                                       574         10
its0,5: nvme0:io4                                       443          8
its0,6: nvme0:io5                                       634         11
its0,7: nvme0:io6                                       360          6
its0,8: nvme0:io7                                       321          6

whereas before it was:

its0,0: nvme0                                            25          0
its0,1: nvme0                                          2991          8
its0,2: nvme0                                          3325          9
its0,3: nvme0                                          2945          8
its0,4: nvme0                                          2809          8
its0,5: nvme0                                          3615         10
its0,6: nvme0                                          2940          8
its0,7: nvme0                                          2750          7
its0,8: nvme0                                          3208          9