amd64 GENERIC: Switch uart hints from "isa" to "acpi"

This causes these hints to be only used to wire device unit numbers
for serial ports enumerated by ACPI but will not create ISA device
nodes if ACPI doesn't enumerate them.  Note that IRQ hints are not
used for wiring so have been removed.

PR:		270707
Reported by:	aixdroix_OSS@protonmail.com, Michael Dexter
Reported by:	mfw_burn@pm.me, Hannes Hfauswedell <h2+fbsdports@fsfe.org>
Reported by:	Matthias Lanter <freebsd@lanter-it.ch>
Reported by:	William Bulley <web@umich.edu>
Reviewed by:	imp
MFC after:	1 week
Differential Revision:	https://reviews.freebsd.org/D45945

sys/amd64/conf/GENERIC.hints

@@ -6,13 +6,11 @@ hint.psm.0.at="atkbdc"
 hint.psm.0.irq="12"
 hint.sc.0.at="isa"
 hint.sc.0.flags="0x100"
-hint.uart.0.at="isa"
+hint.uart.0.at="acpi"
 hint.uart.0.port="0x3F8"
 hint.uart.0.flags="0x10"
-hint.uart.0.irq="4"
-hint.uart.1.at="isa"
+hint.uart.1.at="acpi"
 hint.uart.1.port="0x2F8"
-hint.uart.1.irq="3"
 hint.atrtc.0.at="isa"
 hint.atrtc.0.port="0x70"
 hint.atrtc.0.irq="8"