amr64/gicv3: improve a panic message

[bz]

Summary

Print the device/unit in the panic message for which we cannot get
the MSI device ID to have a clue where to start looking.
While here use func instead of hardcoding the function name.

[emaste]

s/amr64/arm64/ in commit message

LGTM modulo the two comments

sys/arm64/arm64/gicv3_its.c

1127–1128

panics seem to be split on capitalization approach, although the rest of the file seems to use an initial capital so probably leave it so?