asmc: rename wol sysctl to auto_poweron

Older SMC firmware exposed AUPO as a Wake-on-LAN control. On updated
firmware, the key controls automatic power-on when AC power is restored
after a power loss; WoL is handled by the GBE controller instead.
Rename the sysctl to reflect the current semantics. No compatibility
alias is provided as the sysctl has not appeared in any release.

Reviewed by:	ziaee, adrian
Differential Revision:	https://reviews.freebsd.org/D56747

share/man/man4/asmc.4

@@ -1,4 +1,4 @@
-.\"-
+.\"
 .\" Copyright (c) 2007, 2008, 2009 Rui Paulo <rpaulo@FreeBSD.org>
 .\" All rights reserved.
 .\"
@@ -112,6 +112,13 @@ minimum fan speed, the minimum speed and the maximum speed
 respectively.
 .Pp
 All values are in RPM.
+.Sh POWER MANAGEMENT
+The
+.Va dev.asmc.%d.auto_poweron
+sysctl controls whether the machine automatically powers on when AC power
+is restored after an unclean power loss
+.Pq SMC key Dq AUPO .
+A value of 1 enables automatic power-on; 0 disables it.
 .Sh RAW SMC KEY ACCESS
 When the kernel is compiled with the
 .Dv ASMC_DEBUG