asmc: add Wake-on-LAN control via sysctl

Apple Mac systems support Wake-on-LAN from powered-off state (S5/G2) via
the AUPO SMC key.

This change adds a convenience sysctl, dev.asmc.0.wol. This can be
disabled if set to 0 and enabled if set to 1.

The AUPO key is volatile and resets to 0x00 on every boot, so WoL must
be manually enabled before each shutdown to work from powered-off state.
Users need to run: sysctl dev.asmc.0.wol=1 before shutting down the
system. The sysctl is best set to persist in /etc/sysctl.conf.

MFC after: 1 week
Reviewed By: markj, ngie
Differential Revision: https://reviews.freebsd.org/D54439