asmc: output the SMC firmware revision on attach

The SMC firmware revision can prove helpful when determining why the
behavior of a given controller varies from the maintainers' expected
behavior.

This should be a sysctl (eventually), but for now dumping out the
information via device_printf(..) suffices, given that only one
asmc(4) compatible device can exist in an Apple platform at any
given point in time. This will become a sysctl in the future after
additional improvements are incorporated from OpenBSD and NetBSD.

MFC after: 1 week
Obtained from: https://github.com/openbsd/src/ (sys/dev/acpi/asmc.c @ 142d064)
Differential Revision: https://reviews.freebsd.org/D55577

(cherry picked from commit e81e724358e43dcf951e244a9df9df3eaa983fe9)