acpi: Fix trying to register a system memory handler

ACPICA itself already registers its own handler (which works perfectly
fine for our purposes). Furthermore, ACPICA will always fail trying to
register another handler, unless the previous one is explicitly cleared
(which is something the Zephyr code didn't do).

Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>
(cherry picked from commit a0203d1b)